add a webapi and webhooks for a simple http/json-based api

for applications to compose/send messages, receive delivery feedback, and
maintain suppression lists.

this is an alternative to applications using a library to compose messages,
submitting those messages using smtp, and monitoring a mailbox with imap for
DSNs, which can be processed into the equivalent of suppression lists. but you
need to know about all these standards/protocols and find libraries. by using
the webapi & webhooks, you just need a http & json library.

unfortunately, there is no standard for these kinds of api, so mox has made up
yet another one...

matching incoming DSNs about deliveries to original outgoing messages requires
keeping history of "retired" messages (delivered from the queue, either
successfully or failed). this can be enabled per account. history is also
useful for debugging deliveries. we now also keep history of each delivery
attempt, accessible while still in the queue, and kept when a message is
retired. the queue webadmin pages now also have pagination, to show potentially
large history.

a queue of webhook calls is now managed too. failures are retried similar to
message deliveries. webhooks can also be saved to the retired list after
completing. also configurable per account.

messages can be sent with a "unique smtp mail from" address. this can only be
used if the domain is configured with a localpart catchall separator such as
"+". when enabled, a queued message gets assigned a random "fromid", which is
added after the separator when sending. when DSNs are returned, they can be
related to previously sent messages based on this fromid. in the future, we can
implement matching on the "envid" used in the smtp dsn extension, or on the
"message-id" of the message. using a fromid can be triggered by authenticating
with a login email address that is configured as enabling fromid.

suppression lists are automatically managed per account. if a delivery attempt
results in certain smtp errors, the destination address is added to the
suppression list. future messages queued for that recipient will immediately
fail without a delivery attempt. suppression lists protect your mail server
reputation.

submitted messages can carry "extra" data through the queue and webhooks for
outgoing deliveries. through webapi as a json object, through smtp submission
as message headers of the form "x-mox-extra-<key>: value".

to make it easy to test webapi/webhooks locally, the "localserve" mode actually
puts messages in the queue. when it's time to deliver, it still won't do a full
delivery attempt, but just delivers to the sender account. unless the recipient
address has a special form, simulating a failure to deliver.

admins now have more control over the queue. "hold rules" can be added to mark
newly queued messages as "on hold", pausing delivery. rules can be about
certain sender or recipient domains/addresses, or apply to all messages pausing
the entire queue. also useful for (local) testing.

new config options have been introduced. they are editable through the admin
and/or account web interfaces.

the webapi http endpoints are enabled for newly generated configs with the
quickstart, and in localserve. existing configurations must explicitly enable
the webapi in mox.conf.

gopherwatch.org was created to dogfood this code. it initially used just the
compose/smtpclient/imapclient mox packages to send messages and process
delivery feedback. it will get a config option to use the mox webapi/webhooks
instead. the gopherwatch code to use webapi/webhook is smaller and simpler, and
developing that shaped development of the mox webapi/webhooks.

for issue #31 by cuu508

README.md

@@ -33,6 +33,8 @@ See Quickstart below to get started.
   support is limited).
 - Webserver with serving static files and forwarding requests (reverse
   proxy), so port 443 can also be used to serve websites.
+- Simple HTTP/JSON API for sending transaction email and receiving delivery
+  events and incoming messages (webapi and webhooks).
 - Prometheus metrics and structured logging for operational insight.
 - "mox localserve" subcommand for running mox locally for email-related
   testing/developing, including pedantic mode.
@@ -133,12 +135,13 @@ https://nlnet.nl/project/Mox/.
 
 ## Roadmap
 
+- Aliases, for delivering to multiple local accounts.
 - Webmail improvements
-- HTTP-based API for sending messages and receiving delivery feedback
 - Calendaring with CalDAV/iCal
 - More IMAP extensions (PREVIEW, WITHIN, IMPORTANT, COMPRESS=DEFLATE,
   CREATE-SPECIAL-USE, SAVEDATE, UNAUTHENTICATE, REPLACE, QUOTA, NOTIFY,
   MULTIAPPEND, OBJECTID, MULTISEARCH, THREAD, SORT)
+- SMTP DSN extension
 - ARC, with forwarded email from trusted source
 - Forwarding (to an external address)
 - Add special IMAP mailbox ("Queue?") that contains queued but
@@ -447,6 +450,23 @@ messages, for example by replacing your Message-Id header and thereby
 invalidating your DKIM-signatures, or rejecting messages with more than one
 DKIM-signature.
 
+## Can I use mox to send transactional email?
+
+Yes. While you can use SMTP submission to send messages you've composed
+yourself, and monitor a mailbox for DSNs, a more convenient option is to use
+the mox HTTP/JSON-based webapi and webhooks.
+
+The mox webapi can be used to send outgoing messages that mox composes. The web
+api can also be used to deal with messages stored in an account, like changing
+message flags, retrieving messages in parsed form or individual parts of
+multipart messages, or moving messages to another mailbox or deleting messages
+altogether.
+
+Mox webhooks can be used to receive updates about incoming and outgoing
+deliveries. Mox can automatically manage per account suppression lists.
+
+See https://www.xmox.nl/features/#hdr-webapi-and-webhooks for details.
+
 ## Can I use existing TLS certificates/keys?
 
 Yes. The quickstart command creates a config that uses ACME with Let's Encrypt,