Avoid using code only for testing purposes

I believe that the following code should be simplified to only do what it is supposed to do.

https://github.com/bird-house/twitcher/blob/fe51e8f8adcab59daebd4003acbda6dae2d0b5dd/twitcher/models/__init__.py#L53-L82

For testing, the `mock` module should be used to do exactly what the docstring says.
I think that creating a decorator function that does this mocking as required would be the best approach, as it would allow to do something like so:
```
@mock_db
def test_something():
...
```

Start with twitcher/models/__init__.py at the linked lines and read the surrounding docstring and test-related behavior. Check how the current code is used by the tests, then determine the smallest change that keeps production code focused and moves database mocking into a reusable decorator. Done means the documented behavior remains covered by passing tests.

Written by the indexing model from the issue text.