Fix production environment

Due to changes in ProcessDefinition model (now uses search) production or staging environments were broken.

This caused several exceptions like:

• undefined method 'serialize_from_session' for User
• undefined method 'authentication_keys' for User
• unknown attribute 'password' for User.
• and others due to a User model being defined/loaded without the application code (i.e. user.rb) or dependecies (gems) present. In config.cache_classes = true environments these models are never reloaded and thus miss all functionality (see errors above).