Expand to include sound/attendant assignments

[BenShelton]

Dependant on #58 first.

If we allow a user to login with the sole purpose of assigning who is on duties that meeting (such as sound, attendants, microphones, platform) we can include all the logic of not having duplicates and assigning based on privileges as we currently have but just for that section. This would help to avoid schedule clashes more easily, and could also assign based on time too.

This seems unlikely to be helpful before #102 but maybe we can incorporate this separately and before that more major change, if we can assign these roles but not the talk/WT itself that's fine.