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

quickstart.go

@@ -90,8 +90,8 @@ domains with HTTP/HTTPS, including with automatic TLS with ACME, is easily
 configured through both configuration files and admin web interface, and can act
 as a reverse proxy (and static file server for that matter), so you can forward
 traffic to your existing backend applications. Look for "WebHandlers:" in the
-output of "mox config describe-domains" and see the output of "mox example
-webhandlers".
+output of "mox config describe-domains" and see the output of
+"mox config example webhandlers".
 `
 	var existingWebserver bool
 	var hostname string
@@ -563,7 +563,8 @@ WARNING: Could not verify outgoing smtp connections can be made, outgoing
 delivery may not be working. Many providers block outgoing smtp connections by
 default, requiring an explicit request or a cooldown period before allowing
 outgoing smtp connections. To send through a smarthost, configure a "Transport"
-in mox.conf and use it in "Routes" in domains.conf. See "mox example transport".
+in mox.conf and use it in "Routes" in domains.conf. See
+"mox config example transport".
 
 `)
 		}
@@ -774,6 +775,7 @@ and check the admin page for the needed DNS records.`)
 	internal.AccountHTTP.Enabled = true
 	internal.AdminHTTP.Enabled = true
 	internal.WebmailHTTP.Enabled = true
+	internal.WebAPIHTTP.Enabled = true
 	internal.MetricsHTTP.Enabled = true
 	if existingWebserver {
 		internal.AccountHTTP.Port = 1080
@@ -782,6 +784,8 @@ and check the admin page for the needed DNS records.`)
 		internal.AdminHTTP.Forwarded = true
 		internal.WebmailHTTP.Port = 1080
 		internal.WebmailHTTP.Forwarded = true
+		internal.WebAPIHTTP.Port = 1080
+		internal.WebAPIHTTP.Forwarded = true
 		internal.AutoconfigHTTPS.Enabled = true
 		internal.AutoconfigHTTPS.Port = 81
 		internal.AutoconfigHTTPS.NonTLS = true