Set is_staff to False for old board members

One-sentence description

Set is_staff to False for old board members

Why?

This follows from the dicussion in !335 (merged) but it is not clear how and if this should be solved. This issue was created so that we could think about this and discuss this with everyone.

Current implementation

is_staff is set to True for everyone who becomes a board member and is never set to False because board memberships don't end. This is also the reason why committee membership end dates cannot be in the future, since it sets is_staff to False if applicable.

Desired implementation

Do nothing or Set it to False using a Celery task or cronjob for example