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.

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.

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.

* 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

Non-priority production containers will use the master for
writes and a read-replica database for reads. This commit adds
functionality for getting the replica's information from
environment variables and sets up a database routing scheme.

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.

For the base test class which creates an LV-EM user and the
LV-EM observers group, we now also create the basic LV-EM group
and add the user to it as well.

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

Moved request_as_user from GraceDbApiTestBase to GraceDbTestBase
so that it can be used in tests for other apps.

Moved some of the logic in the superevents file list API resource
to a function in the core app, so that it is more easily reusable
across the project.

ModelToDict mixin was supposed to be a generic way of serializing
models, but we decided to go with the serializers from rest_framework
for generic serialization throughout the proejct.

Adding unit test setup classes for managing signoffs and related
permissions.  Also updated permissions assigned to groups
in this test setup.

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.

A new static method is attached to the Log base class,
which uses the VersionedFile method to split a versioned filename
into its filename and version components.

Added explicit methods for versioning a file name and
un-versioning a versioned file name to VersionedFile. These should
be used throughout the code so as to "NRY".

The "full_filename" property for Log objects is now called the
"versioned_filename" to improve clarity.