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, 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//. Test Plan: - 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. Reviewers: btrahan Reviewed By: btrahan Subscribers: epriestley Maniphest Tasks: T7199 Differential Revision: https://secure.phabricator.com/D12230