Rename Feeds::ChangesetCommentsController to ChangesetComments::FeedsController

We usually create nested controllers with the main controller as the module,
and the nested controller as the specialization, e.g. Users::DeletionsController
or Traces::IconsController.

This then leaves the topic of whether the feed resource is plural, and whether we
are then showing a singular feed or showing a list (index) of objects.

The routes are carefully named so that we have `changesets_comments_feed_path` (the
comments feed for all changesets) vs `changeset_comment_feed_path(changeset)` (the
comments for a singular changeset).