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

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

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

Required getting data out of ligolw tables.
Incidentally added processing of Omega results.

Added info for signing up to list on home page.

Fixed a minor type for the MBTA RSS link.
createPayload function was accidentally commented out,
which caused MBTA event creation to fail.  Yow.

New model, EventLog.
Altered event "view" view and event_detail template to show log messages.

Changed uid to graceid, since it's not a uid.
Requires database change:
    ALTER TABLE gracedb_event DROP INDEX uid