Start building UI to customize locations

* Currently allows override of location for entire timesheet
* Gets us started with being able to also set location by project row,
by entry, and by day

The UI is pretty straightforward. Main downside is that it's not yet
clear what location is currently set for a given entry.

#5
Print default location

#5
Adds entry-level editing

You can now edit the location of individual entries

* If an entry is set to something other than the user's default, it
appears blue
* If all of the entries on a timesheet are set to something other than
the user's default, the timesheet-level drop-down will default to that
non-default location (so at a glance you can see what they're all set 
to)


This implementation was incredibly messy relative to my initial 
expectations. In hindsight I wish I'd hacked up a post-back to the
original form instead of doing this with AJAX. Half the new code is in 
place just to keep the client & server side UI in sync.

Fixes #5