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.

New API backend which gets a full X509 certificate, verifies it,
and extracts the subject.  To be used in the cloud deployment
with Traefik.

We had removed the ability for GraceDbModelBackend to do
authentication, but it is relied on for basic auth by the
BasicAuthentication class in django-rest-framework.  So we undo
that change and remove the GraceDbModelBackend class altogether
in favor of its parent, ModelPermissionsForObjectBackend.

Add django-user-sessions package for more easily managing sessions
and correlating them with user accounts.

Complete rework of authentication middleware and backends for
both the web view and the API. There is now a single URL (after
the login page) where the shibboleth attributes are put into
the session and the user is authenticated and a persistent
Django session is created.

Add access_managers and superevent_managers to ADMIN_MANAGED_GROUPS.
They will be removed in the near future, but for now, they should
be included here so that group memberships are not removed by the
ligoauth middleware.

Now require version 2.0.0 of the client and won't allow old
versions without the client header.

Instantiating a second version for the API.  For now, it's identical
to v1, but can be customized as needed in the future.

Moving a few base class throttles and fields from the events app up
one level so that they can be used elsewhere.

Moved events.api and superevents.api into the new api app.
We want to have a versioned API, so these are now in
api.v1.events and api.v1.superevents, respectively. Part of this
change is that we require a custom reverse function (api_reverse)
for handling versioning. Much of the required code changes for this
migration include converting reverses to use this function and
changing relative imports appropriately.

Added utility functions for hiding and exposing logs from
LV-EM and the public. They are triggered by log messages being
tagged or untagged.

Also added utilities for hiding/exposing superevents.

New custom model backend, which includes the table/object
permissions backend and does not authenticate.

New /latest/ page includes events and superevents. We also
move the old events 'latest' page to /events/latest/, changed the
view name, and fixed reverses to that page appropriately.

Activate Django sites app and create a migration which adds the
LIGO.ORG domain name of the GraceDB instance as a Site. This will
be used to build absolute URIs without needed a request object.

Instantiating a new alerts app and moving the alert functionality from
events/alert.py to alerts/old_alert.py. This commit forms a starting
point where the old alert functionality is still in place and we can
rework the alert infrastructure to be more modularized, flexible,
and adapted to superevents.