Matching fixes (issue 3947)

Fixes per issue 3947:
https://code.google.com/p/otwarchive/issues/detail?id=3947

- need to add in a check if there are potential matches but no assignments, which means something's gone wrong, in which case the mod should be able to and told to (1) regen assignments --> (2) regen matches --> (3) contact support. (has automated test now)

- don't let mod send assignments out if there are any unassigned people, make them fix first (has automated test)

- prevent mod from assigning someone who isn't a potential match (this will display the unsaved-but-altered assignment with an error)

Automated tests for all of these, as well as additional automated tests for matching in general.