Implement missing views filters (capacity used, registration status, dates)

I noticed that the following issue for adding filtering based on "registration_entity.status" has been open and frequently requested since 2012:
✨ Enable views filtering on registration_entity.status Needs review

It seems that this issue is often confused with a feature request to be able to filter based on whether registration is open or closed (i.e. whether or not registration is enabled and still accepting registrations).

I also noticed that no one was really mentioning that the "capacity used", "open date", and "close date" fields were missing (the last two can be solved by using Registration Date for these instead).

So, I'm creating this master issue to track these missing filters.