Added properties to superevents which alias other attributes:
  * gpstime = t_0
  * graceid = superevent_id

Adding an endpoint to the superevents API for listing, retrieving,
and modifying superevent access permissions.  Includes views,
a serializer, permissions, urls, etc.

Adding permissions for viewing GOPs attached to a superevent and
the is_exposed field, which will track whether a superevent is
exposed to the public or not.

Adding signoff viewset, serializer, permissions, urls, unit tests,
etc. to the superevents API.

Changed key for response data from 'signoff' to 'signoffs' in
the events API. Historically, there has not been a client method
for accessing this endpoint, so there should not be any issues
with compatibility.

Moving indicator for a test server (config_name) from just below
the navbar to the main heading.

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

Added filtering to recently-updated web-based search for events
and superevents, based on view permissions.

Fixed two bugs in web-based event log creation: first, attempts
to save an EventLog with a null filename, and attempted creation
of the 'lvem' tag with the wrong information, possibly resulting in
an IntegrityError due to duplicate tags.

Deleting unit tests for SupereventForm form, which no longer exists.

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

Many of the classes (permissions, mixins, etc.) in the superevents
API follow a general pattern of
    1. Abstract customized version of a DRF class
    2. Class-specific, customized version of #1
    3. (optional) Implemented, class-specific version of #2
We want all classes in #1 to either be in the root api directory
or the root versioned (api.v*) directory.  #2 and #3 classes should
be in api.v*.superevents. This commit just moves some of the classes
in #1 and #2 around so that they can eventually be used in the events
API.

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.

Add the AnonymousUser from the guardian package to the 'public_users'
group. This will allow us to assign group permissions to
unauthenticated users.

Full unit test suite for user access - making sure that internal and
external users can only see/do what they are supposed to, based on
the defined permissions structure.

We no longer need group object permissions to be assigned
by default for superevent creation since all LVC users will have
table-level permissions to view all superevents.

Custom model, object, and parent object permission classes which
run a function to determine which permissions are required.

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.

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.