Changed the user_to_service mapping table into a model called
ServiceUser. When looking at users who have permission for a folder
we are only interested in users for a particular service, not all users,
so we can use the ServiceUser model to access folder permissions.
Added a user_folder_permissions table which contains the service_id,
user_id and template_folder_id. There are links between
user_folder_permissions and TemplateFolder, and between
user_folder_permissions and ServiceUser.
