Convert comments to be revisionable

Seems to be some failures in the MR2872

Also the update hook will need upgrade tests.

Probably should update comment_user_cancel() so that revision_uid is re-assigned to zero (0) when the author's account is cancelled. I recently ran into an issue where the comment was re-assigned but not the revision. See node_user_cancel() as an example:

node_mass_update($vids, [
  'uid' => 0,
  'revision_uid' => 0,
], NULL, TRUE, TRUE);