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.