Draft: Enforce typing

Add mypy to build process to validate if type hints are respected.

Few things that are still not done:

• generic type hints don't work in Python 3.6 (if we decide to use it we need to drop support for python 3.6)
• Generic types for django ModelForm/Field does not work
• Generic types for django ListView/CreateView/UpdateVie does not work
• get_user_model() is not discovered properly
• screening number should be changed into array - current implementation enforce violation of some types, and is hard to maintain (sorting cannot be done in db)
• find a way to properly type django magic:

    def image_img(self) -> str:
       return '<img class="flag-icon" src="%s" />' % self.image.url
    image_img.short_description = 'Flag icon'  # type: ignore