Adding trip ticket comments

* Initial trip ticket comments scaffolding and routing
* Migrating to add new trip ticket comments table
* adding associations to trip ticket comments
* Adding tests for site_admin ability on trip ticket comments
* Removing user association from trip ticket comments. Will use audited gem to track who created it, and only users of the originating provider should be able to create comments
* Adding provider_admin abilities on trip ticket comments
* Adding scheduler abilities on trip ticket comments
* Adding User#display_name instance method
* Adding back user_id column to trip ticket comments
* Rerouting trip ticket comments to get around complication with CanCan and load_and_authorize_resource
* Wiring up rest of views for trip ticket comments