Ref T2222.
- Removes DifferentialTasksAttacher, which has had no callsites for a very long time.
- Moves differential.getrevisioncomments off DifferentialCommentQuery.
- Moves Releeph churn field off DifferentialCommentQuery.
- Removes dead code in DifferentialRevisionViewController.
- Removes DifferentialException (no references).
- Removes DifferentialRevision->loadComments() (no callsites).
- Removes DifferentialRevision->loadReviewedBy() (all callsites updated).
- Removes DifferentialCommentQuery (all callsites updated).