add config options to disable a domain and to disable logins for an account

to facilitate migrations from/to other mail setups.

a domain can be added in "disabled" mode (or can be disabled/enabled later on).
you can configure a disabled domain, but incoming/outgoing messages involving
the domain are rejected with temporary error codes (as this may occur during a
migration, remote servers will try again, hopefully to the correct machine or
after this machine has been configured correctly). also, no acme tls certs will
be requested for disabled domains (the autoconfig/mta-sts dns records may still
point to the current/previous machine). accounts with addresses at disabled
domains can still login, unless logins are disabled for their accounts.

an account now has an option to disable logins. you can specify an error
message to show. this will be shown in smtp, imap and the web interfaces. it
could contain a message about migrations, and possibly a URL to a page with
information about how to migrate. incoming/outgoing email involving accounts
with login disabled are still accepted/delivered as normal (unless the domain
involved in the messages is disabled too). account operations by the admin,
such as importing/exporting messages still works.

in the admin web interface, listings of domains/accounts show if they are disabled.
domains & accounts can be enabled/disabled through the config file, cli
commands and admin web interface.

for issue #175 by RobSlgm

doc.go

@@ -65,10 +65,14 @@ any parameters. Followed by the help and usage information for each command.
 	mox config describe-static >mox.conf
 	mox config account add account address
 	mox config account rm account
+	mox config account disable account message
+	mox config account enable account
 	mox config address add address account
 	mox config address rm address
-	mox config domain add domain account [localpart]
+	mox config domain add [-disabled] domain account [localpart]
 	mox config domain rm domain
+	mox config domain disable domain
+	mox config domain enable domain
 	mox config tlspubkey list [account]
 	mox config tlspubkey get fingerprint
 	mox config tlspubkey add address [name] < cert.pem
@@ -967,6 +971,26 @@ All data for the account will be removed.
 
 	usage: mox config account rm account
 
+# mox config account disable
+
+Disable login for an account, showing message to users when they try to login.
+
+Incoming email will still be accepted for the account, and queued email from the
+account will still be delivered. No new login sessions are possible.
+
+Message must be non-empty, ascii-only without control characters including
+newline, and maximum 256 characters because it is used in SMTP/IMAP.
+
+	usage: mox config account disable account message
+
+# mox config account enable
+
+Enable login again for an account.
+
+Login attempts by the user no long result in an error message.
+
+	usage: mox config account enable account
+
 # mox config address add
 
 Adds an address to an account and reloads the configuration.
@@ -992,7 +1016,13 @@ The account is used for the postmaster mailboxes the domain, including as DMARC
 TLS reporting. Localpart is the "username" at the domain for this account. If
 must be set if and only if account does not yet exist.
 
-	usage: mox config domain add domain account [localpart]
+The domain can be created in disabled mode, preventing automatically requesting
+TLS certificates with ACME, and rejecting incoming/outgoing messages involving
+the domain, but allowing further configuration of the domain.
+
+	usage: mox config domain add [-disabled] domain account [localpart]
+	  -disabled
+	    	disable the new domain
 
 # mox config domain rm
 
@@ -1003,6 +1033,23 @@ rejected.
 
 	usage: mox config domain rm domain
 
+# mox config domain disable
+
+Disable a domain and reload the configuration.
+
+This is a dangerous operation. Incoming/outgoing messages involving this domain
+will be rejected.
+
+	usage: mox config domain disable domain
+
+# mox config domain enable
+
+Enable a domain and reload the configuration.
+
+Incoming/outgoing messages involving this domain will be accepted again.
+
+	usage: mox config domain enable domain
+
 # mox config tlspubkey list
 
 List TLS public keys for TLS client certificate authentication.