Implement filters

[henhuy]

This PR implements back- and frontend for filters.

[henhuy]

Hey @josihoppe, thanks for your implementation of forms - looks already nice! With my newest updates to dev-branch, we ca now adapt filters from real django models. So next steps would be:

• [x] Merge dev into this branch
• [x] Build filter form with dynamic fields read from model (read filters from currently used datamodel*)
• [x] Scenarios are selected beforehand in separate view and are accessed from GET attributes as list (instead of a multi select input on same page)
• [x] filter options/choices are read from queryset, something like this (pseudo-code):

  scenarios = scalar_model.objects.filter(scenarios__in=scenario_list)
  tags = scenarios.values("tags").distinct()
  regions = ...

  and set directly as choices while setting up fields in __init__ function

After implementing above steps, I will meld your filter selection with my current graph creation!

*: Here is how I get currently used scalar datamodel: https://github.com/OpenEnergyPlatform/django-comparison-dashboard/blob/cbeaee9602ed2917b280952229de6e74da65c8c4/django_comparison_dashboard/preprocessing.py#L47 and here an implementation of dynamic fields in a form: https://github.com/rl-institut-private/digiplan/blob/b60afdefdab7bd81b4df3a4dc8e291368f8103af/digiplan/map/forms.py#L42 (you can simply add a dict containing field name as key and Field class as value to self.fields)