🔖 concrexit v54.2

This fixes a bug where members who have already renewed their membership were not seen as current members because their latest membership is not yet active. Now, we check for a current membership. New members, who register for Thalia in August, now get a membership that starts on the day their registration is completed. Previously, these got a membership that would start on the 1st of September, and those would still be able to act as if they had a current membership in August due to missing checks on the start date of memberships.

See #3777.