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.

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

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

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