Added a 'get_subclass' method to the Event model which is used to
get the CoincInspiralEvent, GrbEvent, etc. corresponding to that
Event.

Reorganized some utility methods in the alerts app, necessitating
some changes to imports in various places.

Added a method to the Superevent model which returns a list of groups
who have group object permissions for that particular superevent.
Could be useful in the future.

It's too complex and doesn't make much sense to make the superevent
GOP serializer a group serializer, even though it makes the display
a bit nicer.  Instead, we view and serialize the GOPs themselves.

Moved context variables and calculations for determining whether
the confirm-as-gw form should be shown on the superevent detail
web page from the superevent detail view class to a separate mixin.

Updated the web view for the superevent file list to be a
class-based view which uses view permission-based filtering. Also
updated the file links on the list page to point to the file download
page on the API so as to reduce redundancy.  Removed the old file
list and detail web views.

Added some information about old/new IDs to the log message which
is created when a superevent is "upgraded" to a GW.

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.

Created a new mixin for filtering a queryset for a detail view
by using the user's view permissions and applied it to the
superevent detail view.

Moved the javascript for this button to a new location in the main
JS file and added button enable/disable code to prevent "too fast"
or multiple clicks.

The 'confirm GW' button on the superevent web detail page now
uses AJAX to make the request to the corresponding API endpoint.
As a result, the separate view function for this endpoint in the
superevents app is no longer needed, so we have deleted it.

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.

Removing old web views for superevents app which are no longer
used and cleaning up imports.

Since we use direct foreign keys for superevent permission tracking,
there is no need to delete GropuObjectPermissions when we do a
"purge" delete.

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

Updating and adding utility functions for exposing/hiding
superevents and managing signoffs.

Changing signoff form and expose/hide button on the superevent
web display to make an AJAX request to the relevant API endpoints.
This way, we don't have to maintain separate permissions and views
for web resources.

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.