Fix a major regression in comment ordering.
Review Request #8739 — Created Feb. 11, 2017 and submitted
A recent change going into 2.0.26 and 2.5.8 broke comment ordering, causing them to be ordered by ID and not timestamp. This could greatly alter reading and composing a review, along with API results. The cause was the subclasses of BaseComment introducing a Meta that overrode the one from the parent, instead of subclassing it. This fixes that by subclassing the parent's meta, inheriting the ordering. It also removes the app_labels, which were already provided by the parent.
Unit tests pass.
No longer saw the comment ordering problem in tests.