Move common logic used when creating exercises to the model

--HG--
extra : amend_source : a5e3dc39de561fbc8682002a82c5f698507606a0
Refactor common logic used for user-submitted models

- New abstract class with simplified status (accepted, pending, declined)
- New manager with custom filters (.accepted(), etc.)

--HG--
extra : amend_source : e8a8c9a252537e03dbe235c056645b1c29f42cb4