new linter: All forms must inherit from ModelForm

[atodorov]

In general almost all forms in Kiwi TCMS must inherit from ModelForm. This will have the benefit of simplifying the application and clean up some duplicate code. It will also help discover functions/views which allow sending fields that are not part of the model itself (there was such a case with TestCase.update API method).

Kiwi TCMS needs a custom kiwi_lint/ checker which will discover all form classes which do not inherit from ModelForm and warn about them.

There will be a second bounty for fixing the offending forms b/c there would be quite a few I expect.

Definition of Done:

• new checker class under kiwi_lint/ which implements the necessary pylint machinery
• Can be relatively simple checker. It should be sufficient to raise a warning when a function call contains a parameter with the name label. If this is too generic we can refine it later
• All code review passed & PR merged

Here's me explaining how to write pylint plugins:

• https://www.youtube.com/watch?v=3CkSKUNMLJc
• https://www.youtube.com/watch?v=D1wLFyazTRM
• checkout node_is_subclass from pylint-django (which we use anyway) https://github.com/PyCQA/pylint-django/blob/master/pylint_django/checkers/forms.py

This issue is part of Kiwi TCMS open source bounty program. For more information see the link(s) in bounty-program milestone