Move models to respective test modules

What needs to be sorted out:

1. What to do with models that are referenced in multiple test modules. Cloning them into each test module feels bad because of <i>unDRYing</i> the code massively.

2. Same as the 1st, but relating to `RESTFrameworkModel`, since multiple models use it from all over the place, maybe remove it and set app label for all them from a `TESTS_APP_LABEL='tests'` somewhere in setting i.e. `conftest.py` or `utils.py`.