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).

app/controllers/changeset_comments/feeds_controller.rb

@@ -1,16 +1,16 @@
-module Feeds
-  class ChangesetCommentsController < ApplicationController
+module ChangesetComments
+  class FeedsController < ApplicationController
     before_action :authorize_web
     before_action :set_locale
 
-    authorize_resource
+    authorize_resource :changeset_comment
 
     before_action -> { check_database_readable(:need_api => true) }
     around_action :web_timeout
 
     ##
     # Get a feed of recent changeset comments
-    def index
+    def show
       if params[:changeset_id]
         # Extract the arguments
         changeset_id = params[:changeset_id].to_i