Cleanups.
kegadmin: Clean up units editor.

Also closes #279.
Support some other standard keg sizes.

Closes #318.
core: Capture time zone with session data.

Previously, statistics were incorrectly computed against
the stored timezone (UTC), which was most evident in
day-of-week calculations.

Since the time zone setting can be changed by a manager,
it must be stored along with drink data.  For now, we do
so only on the session record.

Closes issue #199.
Update demo data.
core: Allow timestamp to be overridden in backend.
core: Fix backup tests.