Major set of improvements

[PetrDlouhy]

I have started transformation of django-admin-smoke-tests to my new project. With it I made whole lot of improvements to it's code.

The particular features are relatively well separated into separate commits, but I am not able to keep them in separate PRs without too much overhead. Especially if the essential PRs to build upon don't get merged any time soon.

@SeanHayes If you give me access to this repository, I will maintain the project here. Otherwise I will have to move to my own fork and make the changes also on PyPI

The new features are:

• Upgrade to newest Django/Python (Update to work with supported Django versions (2.2-4.1), use GitHub actions for testing #17)
• Creating of test models with model_bakery (create some test data with model_mommy #18)
  • Possibility to define Recipes for the bakery
  • Using MPTT_ALLOW_TESTING_GENERATORS=True when creating models
  • Create models in setUp() rather than on multiple places in code
  • Option strict_mode which doesn't allow failures in model creation
• Removal of default fixtures (remove explicit definiton of django_admin_smoke_tests fixture, update README #21)
• Running tests through subTest() (Run test for every ModelAdmin in subtests #23)
• Many fixes that highers chance that the tests will pass:
  • Planting SessionMiddleware and messages storage into requests
  • Strip search_fields prefixes when testing specified fields
  • Try also *_set attribute when testing specified fields
• Much more options for changing behavior of AdminSiteSmokeTestMixin by method overrides:
• Don't ignore PermissionDenied, rather test whether user has permission
• The only_modeladmins option
• Much wider options what to put inside only_modeladmins and exclude_modeladmins (model admin name string, model admin full path string, model admin class)
• Testing filters: running changeview with various parameters based on list_filter

There are still few things needed to finish this:

• Add thorough documentation that will include:
  • override options
    • create_superuser()
    • get_url()
    • create_models()
  • only_modeladmins, strict_mode, recipes_prefix options
  • model creation and Recipes
  • manual model creation
  • using MPTT models
• Use assertContains() at least to some element present in all general admin pages, test it in separate method to allow overridability
• Test admin index page
• Possibility to run subset of tests through env variables, add docs
• Fix or document errors produced by bakery generated models and if they can be somehow fixed to minimize needed configuration of the testing
  • Field is nullable althoug save() doesn't support it
  • Field is not supported by baker
  • FileField
  • Field is too long (field lengthened in save() method)
  • Field doesn't have default
  • Model is created during related model creation
  • Field is unique

Ideally move bakery field generators to the appropriate apps:

• django-avatar
• django-money (add field generator for model bakery django-money/django-money#687)
• django-extensions
• jsonfield
• phonenumber field