Fix `BroadcastDayList` to handle all broadcasts on invalid input

The implemented refactoring from #317 (closed) is not working stable. I needed some time to figure out whats happening, but I was not able to find a proper working solution, so needing help here :(

• I was wondering, why I can not open the broadcast list ("Programmplan") again and why I am resulting in an blocking, endless loading. Regardless of language.
• I figured out, that the side tries to load and render all 55.000+ broadcast by enabling SQL debugging and adding a len(). I was unable to correct this as enduser and was trapped.
• I had also some trouble getting a stable behavior, as it seems that this just appeared on my already logged in browser, not on a new logging in one.

It is a combination of two things

1. get_context_data() returns unfiltered broadcasts

The following line from the get_context_data in class BroadcastDayList(FilterView) seems to not-filter, when the input is not correct. This can be triggered for example by http://localhost:8000/manage/broadcasts/?date=15.11.2022FOO. The self.filterset.is_valid() returns false and the full context (containing all broadcasts, as nothing is filtered) is returned. At the end, the page is rendered for a loooong time :)

def get_context_data(self, **kwargs):
    context = super().get_context_data(**kwargs)

    if not self.filterset.is_valid():
        return context

2. Session Handling

I was not knowing about the sessions functionality in the code, now I learned some stuff. But: The session is also storing the wrong date 5.11.2022FOO without testing and when I come back applying this wrong, so I am not able to leave the cage anymore.

def get(self, request, *args, **kwargs):
    if self.request.GET:
        self.request.session[SESSION_BROADCAST_DAY_KEY] = self.request.GET
    return super().get(request, *args, **kwargs)

Approaches

• Prohibit the return of the full broadcast list hard (as fallback).
• Return the current datetime.now().date() if reading of a proper date from request.GET failed
• Validate session data of SESSION_BROADCAST_DAY_KEY before usage and clean if validation fails.

Further Aspects

• I was struggeling by implementation attempts in manipulating the queryset. I can overwrite the date inserted to get_weekday_filter_context(), but I still get not queryset manipulated to use the current day. I also tried making a redirect, but I was unsure where I should add it.
• The SESSION_BROADCAST_DAY_KEY is also used in program/views/episode.py, there may be the same problems too?