Allow revisions to be edited from Maniphest

Summary:
You can currently attach tasks to revisions from Differential, but not revisions
to tasks from Maniphest. Allow editing from either side.

This logic is kind of tricky but the alternative was massive code duplication.

Test Plan:
Added and removed revisions from maniphest. Added and removed tasks from
differential.

This should have no impact on the Facebook install since none of this is used
there.

Reviewed By: aran
Reviewers: tomo, tuomaspelkonen, jungejason, aran
CC: aran, epriestley
Differential Revision: 288