Ref T7199. This prepares for an exciting new world of more powerful "!action" commands. In particular:

• We parse multiple commands per mail.
• We parse command arguments (these are currently not used).
• We parse commands at the beginning or end of mail.

Additionally:

• Do a quick modernization pass on all handlers.
• Break legacy compatibility with really hacky Facebook stuff (see T1992). They've theoretically been on notice for a year and a half, and their setup relies on calling very old reply handler APIs directly.
• Some of these handlers had some copy/paste fluff.
• The Releeph handler is unreachable, but fix it in theory.

• Sent mail to a file; used "!unsubscribe".
• Sent mail to a legalpad document; used "!unsubscribe".
• Sent mail to a task; used various "!close", "!claim", "!assign", etc.
• Sent mail to a paste.
• Sent mail to a revision; used various "!reject", "!claim", etc.
• Tried to send mail to a pull request but it's not actually reachable.