Sort by number of enrollments

The users need a convenient way to find overlapping affiliations.

This isn't easy, as it depends on the time.

Having the possibility of listing individuals based on criteria like the ones listed here should help:
- by the number of current affiliations
- by the maximum number of overlapping affiliations at any time (maybe stored with the individual?)
- by the number of affiliations (overlapping and not. This is easier to calculate, but rough for this purpose)