GroupObjectPermission and data directory cleanup should only happen
*after* the event or superevent row is removed from the relevant
table.

A web view has been created which allows EM advocates to disable
or enable pipeline submissions. This is to prevent misbehaving
pipelines from submitting bad information. The mechanism works by
preventing events from being submitted to a given pipeline, not
by revoking certificates or removing user account permissions.

Previous arrangement was not including zero-valued fields. We now
include all non-None fields.

New model functionality, better forms and views.

As shown here: emfollow/userguide!23

As requested here: emfollow/gwcelery!359

The settings for gracedb-playground are no longer on a separate
branch and are just included in master. They can be selected
simply by settings the DJANGO_SETTINGS_MODULE environment variable
appropriately.  This should be much easier to maintain than a
completely separate branch.

Previously if an Event did not have a Search, no 'search'
key was included in the serialized event dictionary. Now we include
one and set it to None if the Event has no Search.

The format (which is staying the same) is like 'ID-N-TYPE', but for
retractions, TYPE was like "TYPE-OF-PREVIOUS-VOEVENT"-Retraction
(ex: Update-Retraction).  Now we just use TYPE=Retraction.

VOEvent IVORNs now use ivo://gwnet/LVC instead of
ivo://gwnet/gcn_sender. Also use the default superevent ID everywhere
in a VOEvent, including filename, even if it has a GW ID.

Some additional code cleanup, as well.

Make the event view 403 pages more specific (for event views only),
but more generalized (standard message in the template, not taken
from the context).  Also fix up some unit tests which expected the
old behavior.

Move a number of scripts which were managed in a separate repo
into management commands.  This way they can be more easily executed
either manually or by cron jobs.

Add some information to VOEvents and rearrange some things:
  * Add EM Bright information (BNS, NSBH, BBH, Terrestrial) in a
    Group in the What section
  * Put ProbHasNS and ProbHasRemnant in a Group and rename to 'HasNS'
    and 'HasRemnant'

See https://git.ligo.org/lscsoft/gracedb/issues/77