Desk worker sign in/out

[Jacob Hurwitz]

Have some notion for a desk worker to say "I'm starting to work now" and "I just finished working."

This fixes the problem of determining whether to show the student or desk UI to a person who's both a student and a deskworker.

Other potential benefits include: We can log how long each deskworker has worked, possibly removing the need for other timesheets (which I think some dorms have).

Possible problems: Random desk timesheets are weird because you get paid extra for time doing mail at desk. Also, if you link this to timesheets but also need to sign in before being shown any desk features, it now becomes impossible for deskworkers to do desk-type jobs when they are off the clock. (This could be good or bad -- we'd need to think about it.)