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/