Lots of failovers seemed to be leaving lots of open connections
to the LVAlert server, so it seems that the connection wasn't being
closed properly. Looks like abort() is the method that should
be called instead of disconnect().

Have to use mock to properly modify throttles for unit testing
since 'override_settings' doesn't really work with the API
settings used by rest_framework.

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.

All throttles now use a database-backed cache since that is the
only way to do centralized throttling (important for production
deployment with multiple workers). We also add default throttles
for anonymous users for the entire API.

Various javascript functionalities and forms in the web interface
make calls to the API to get or process data. The current
Apache configuration (verify a certificate IF presented on /api/*)
causes pop-ups for users who have certificates in their browsers
if they visit any pages which have these functionalities. So we have
changed all of those URLs to use /apiweb/ for now to prevent the
certificate challenge.  This will be resolved in the future by
getting away from certificate-based authentication.

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.

Not sure why this started happening all of a sudden since we have
been using 2.5.2 for quite a while.  But trying to pip install
python-ldap when building a Docker image keeps giving a
UnicodeDecodeError. Updating to version 3.1.0 seems to resolve it.

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.

Containerized versions of the service will now get their LVAlert
credentials from the environment.  Also add a script for
processing environment variables and starting LVAlert overseer.