Detect tasks referenced in commit messages and either link or update the mentioned tasks. Refs T945

Summary: This takes the place of D2721 which I am going to abandon.

Test Plan:

• Make a commit with "Closes T###" in the summary field. See that the mentioned task gets closed.
• Make a commit with "refs T###" in the summary. See that it gets added as a related commit via edges.

Reviewers: 20after4

Reviewed By: epriestley

CC: aran, Korvin, champo

Maniphest Tasks: T945

Differential Revision: https://secure.phabricator.com/D3466