Generalize roles and add an "admin" role
Fix a couple edge cases related to empty orders

These were causing the ActiveAdmin UI to blow up.
Use location name on admin user edit form

Otherwise we take forever pulling together an enormous list of locations
for a mondo dropdown
Bugfix in the new generalized role stuff
Convert admin auth to be role-based

Replaces/obsoletes the old HTTP digest auth and admin password. Not
needed now that we have HTTPS, and requiring user login and HTTPS is
more secure anyway.
One more attempt to fix flaky tests

Time comparisons are not robust apparently; I keep getting 1-microsecond
discrepancies, e.g.:

 <Tue, 23 Apr 2013 18:23:59 PDT -07:00> (258252000[ns]) expected but was
 <Tue, 23 Apr 2013 18:23:59 PDT -07:00> (258251000[ns]).

I'm rounding to seconds here as per
http://blog.tddium.com/2011/08/07/rails-time-comparisons-devil-details-etc/