Add model metaclass factory and fix/extend some documentation

This PR is a bit larger than the last. Mainly, it introduces a factory function for a base metaclass the sameway there is one for the original form class. Additionally, I fixed some documentation flaws (e.g. broken URLs) and added some more documentation for the factory functions and the resulting classes (I also fixed a docstring that was leftover from my last PR and was wrongly assigned to the class instead of `get_session`. There, I also reworked the API documentation to also include all elements that already have valid docstrings so it can be read directly in the documentation.

Finally, I added a new test for the new metaclass. This tests the metaclass by creating default forms from it (inspired by the other tests) and checks that inheritance works properly (in fact the first commit, fa9f9be, had a bug this test found). I also extended some existing tests to work with all three options now instead of the default.

Concerning the tests: I am not familiar with the testing environment, so I am not a 100% sure I properly tested everything here correctly. Please review my testing and check if I could improve it somehow.