This commit adds tests for userdbs.views.AddUserDatabase, the view
itself has been modified so that it is only reachable when the hosting
package actually has database options available.
- modify userdbs.models.UserDatabase.delete to perform deletion of database
user if it has no other databases assigned
- implement userdbs.views.DeleteUserDatabase
- add URL pattern 'delete_userdatabase' to userdbs.urls
- add template userdbs/userdatabase_confirm_delete.html
- add link to 'delete_userdatabase' on hosting package detail page
- add changelog entry
