R1:0d99c84bd78e

Modernize email command parsing

Summary:
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…