Add a commit_id field to ReviewRequest

This change adds a new field called 'commit_id' to the ReviewRequest model. This
field obsoletes the 'changenum' field. There's also a property called "commit"
which will aggregate together the old "changenum" field and the new "commit_id"
field.

This new field is a 64-character string instead of an int, in order to handle
SCMs that don't use numeric revisions (like git).

Testing done:
- Tested the migration with a database that had review requests with both null
  and non-null changenums
- Ran unit tests
- Did some smoke testing of various pieces of UI that deal with changenums

Reviewed at http://reviews.reviewboard.org/r/4237/
Fix the state of the dashboard after refreshing.

The dashboard state was broken after its first load. The columns were no
longer interactive, nor was the Edit Column icon. The relative timestamp
columns showed absolute times.

We now set up the state again once we've completed a load.

Fixes bugs #2993 and #2994.
Merge branch 'release-1.7.x'