new model caching: refactoring Structureable.

In `StructureableMixins::Roles`, now implement the `fill_cache` and the
`delete_cache` method. Using the Ruby-2 `prepend` mechanism, the
`super` method can be called within these module methods.
Applying the new model caching to the Group model.

Trello:
https://trello.com/c/V6polddr/660-neues-konzept-fur-model-caching