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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.

Some additional code cleanup, as well.