Maniphest - add support for !assign command

Summary:
also try to centralize some of the command parsing logic. note that differential is still an exception here. it uses a whitelist-style regex. i think long-term we should have this for every app but changing it seemed too big for this diff.

Fixes T3937.

Test Plan:
echo '!assign btrahan' | ./bin/mail receive-test --as xerxes --to T22 ; echo '!claim' | ./bin/mail receive-test --as xerxes --to T22

unit tests passed, though my new one is silly

Reviewers: epriestley

Reviewed By: epriestley

CC: Korvin, epriestley, aran

Maniphest Tasks: T3937

Differential Revision: https://secure.phabricator.com/D7307