GetParentObject and SupereventParentMixin are now full viewsets
rather than just mixins.  They also get the parent object during
the initial phase of request processing, cache it, and are able to
check object permissions on the parent.

get_superevent_by_date_id_or_404 no longer takes a request
object as an argument (wasn't using it anyway).

I had been mistakenly using lookup_field for the lookup_url_kwarg.
This commit fixes that mistake to use both field properly. It also
lets us remove the customized GetObjectDefault mixin that was
trying to deal with this behavior.

Assigning permissions for viewing, creating, exposing/hiding, etc.
for superevents, logs, labels, etc. in the superevents app.

Created the 'access_managers' and 'superevent_managers' groups. These
groups will be given appropriate permissions for managing external
access to gracedb and creating/updating superevents, respectively.

The built-in DjangoObjectPermissionsFilter from DRF doesn't
accept global (table-level) permissions as higher-level than
object permissions. This filter replaces the built-in one
and sets the 'accept_global_perms' argument to True so that
global permissions are accepted when filtering objects based on
user view permissions.

Adding custom permissions for Superevent and Log objects
in the superevents app.  We also add *direct* foreign keys to
the permissions objects for performance.

Since we are still developing on the superevents branch,
it makes sense to squash the migrations set into a single one,
just to keep things nice and clean.

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".

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

New auth backend for permissions. Basic ModelBackend returns false
if user has table-level permissions, but an object is passed in.
This new backend establishes that if the user has table-level
permissions, those supersede the lack of row-level permissions on
a given object.

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

Removing use of the router for defining superevent list and
detail URLs, so that all of the possible URL/method combinations
are explicitly defined.

Since we use the preferred event to construct much of the VOEvent
for a superevent, we have to select the event's subclass (note that this
is done for events by the event_and_auth_required decorator, which calls
Event.getByGraceid).  This was preventing certain subclass-specific
attributes from being added to the VOEvents, like ProbHasNS,
ProbHasRemnant, etc.

Superevent now always have a letter suffix - remove the logic
which gives the first superevent on a date no letter suffix and
allows it to be looked up without the letter suffix even after
it receives one.

Changing SupereventID -> GraceID and SupereventPage -> EventPage
for consistency with old VOEvents.

Update alert code to determine whether the message should go to
the production, test, or mdc superevent nodes.

Superevent categories are now exposed by the API root, for
use in gracedb-client.

Superevent category is now queryable.  Also set default queries
to not return Test or MDC superevents.

Updated superevent creation and update serializers to handle
superevent categories.  Also updated utils to accept the category
argument and do some checks.

Created ChoiceDisplayField - same as ChoiceField, except its
to_representation() used the display value instead of the key.

Adding utils to Superevent and Event models for comparing
categories and deciding if an Event is the correct category to be
part of a superevent.

Superevents can now be of type 'Production', 'Test',' or 'MDC'.

Models which inherit from AutoIncrementModel can now specify
more than one constraint.  Needed for superevent categories.

Link to correct file versions in superevent log comments

Fixing issue where users could cause superevent date ID lookup
to fail by providing date strings which couldn't be parsed by
datetime.datetime.strptime (ex: '181309', month=13). Now we raise
a custom exception in the class method and catch it in
get_superevent_by_date_id_or_404.

Replacing redundant logic for superevent queries to just use
Superevent.get_filter_kwargs_for_date_id_lookup so as to "NRY".