Data migration to populate new database columns for existing
VOEvents. We have to read the files in and try to extract the
parameter data, handling all of the changes to the file format
which have occurred over time.

I did a lot of testing by copying over the production database
and associated VOEvent files for all non-Test, non-MDC events
and superevents to make sure we are accounting for all changes
in VOEvent (file) schema that have happened over time.

We can change a few things about how the VOEvent file building
works now that all of the relevant parameters are stored in the
database.

There are several fields that the user provides to create a
VOEvent through the API, which are used in generating the VOEvent
file, but are not stored in the database. Let's fix this!!

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.