Use the 'sync' worker still due to issues with handling file
uploads when gunicorn uses at least 6 but possibly more than 2
workers.

Suggested keepalive time is 1-5 seconds for standard deployment,
but it should be much higher for deployments behind a load
balancer. We are hoping this might resolve the SSLEOFErrors which
have been seen on the AWS deployment.  Traefik's keepalive time
is hard-coded to 90 seconds so we want gunicorn's to be longer than
that, so we set it to 100 s.

Increase number of attempted restarts for LVAlert Overseer in
a container from 3 to 100. With only three attempts, it doesn't
even consistently get going again without going into a FATAL state
when the openfire server is simply restarted.

These settings were previously handled in settings/container/production.py,
but we want to do some testing and I don't see any issues with having
them in the base container settings (settings/container/base.py).

Upgrade gunicorn to 19.9.0 and switch to an asynchronous
worker class

Instead of looping over EmailMessage.send() calls, we now get the
email backend and send a list of messages. Note that the backend
sends the messages individually still but this cuts out some of the
overhead.

Prevents server errors from being generated when the filename is
"bad" and when the user specifies a non-integer version. This is
for superevent files; event files are handled separately, although
this should be unified at some point.

We use get_or_create now to add labels to a superevent in order
to prevent IntegrityErrors when a user makes the same request
too rapidly and both sneak by the initial check.

overseer seems to always exit with an exit code of 0, which means
that 'autorestart=unexpected' does not restart it. So we set it
to 'true' instead.

Not feasible to use multiple DBs for read or write operations at
present. We're going to have to revisit this at some point. We leave
most of the code in place but commented out.

Add capability for passing kwargs to AlertIssuer classes'
issue_alerts() method.  We take advantage of this in the update
alerts for checking for FAR changes and NSCAND state changes.

Add a whole suite of tests for phone and email alerts. Small update
to a few unit tests for alert views.

Small fixes to the main "index" view for alerts. Move the
manage_password page for LV-EM observers to the ligoauth app and
show a link to it from the navbar to those users.

* Filter available contacts for a notification to be verified
  contacts belonging to the user
* New better code for handling label queries
* Some better help text for notification forms
* Remove old notification forms and views