cli_tag, non-functional form drop-down, show tags on event detail page.

Ditched drop-shadows in top menu bar.  (ugly!)

Version 1 implemented -- has cleaner error handline.

Added a 404.html template.
gracedb/view raises Http404 when event is not found.
Event.getByGraceId should now never return None.

On failures, users will get nicer looking messages and admins will get email.

REQUIRES db schema mods in upgrade-3-4.sql

Changed display of all date/time data to be UTC.
GPS times display UTC times when cursor hovers over them.

Note that authorized followup user names are hardcoded into views.py