Use a lambda in order to pass parameters in before_actions

This avoid calling methods on a new instance of the controller, and
therefore allows these methods to be marked as private.

As a bonus, rubocop can now parse them and warn when they refer to
actions that don't exist in the controller.

app/controllers/changeset_comments_controller.rb

@@ -4,7 +4,7 @@ class ChangesetCommentsController < ApplicationController
 
   authorize_resource
 
-  before_action(:only => [:index]) { |c| c.check_database_readable(true) }
+  before_action -> { check_database_readable(true) }
   around_action :web_timeout
 
   ##