Clean up various pieces of dead/obsolete Differential code

Summary:
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).

Test Plan: Mostly a lot of grep.

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T2222

Differential Revision: https://secure.phabricator.com/D8476