Membership-algorithm update

[sema]

The old membership-algorithm can't handle the high frequency of events being provided today (remember, at the time this algorithm was written we only had two events a year, and it relies on this invariant).

The algorithm right now attaches a membership payment to the first event a member signs up for. If another event is created at a date before it, and the user attends this one, the algorithm will require the member to create another membership payment. This results in the algorithm asking for membership payments at times when it is not required.