We can change a few things about how the VOEvent file building
works now that all of the relevant parameters are stored in the
database.

We've had some bad cases where a log entry is created, but the
file is not saved due to filesystem errors. So we put the file
creation in a try-except block and if it fails, delete the log
message that was just created before re-raising the exception.

We use get_or_create now to add labels to a superevent in order
to prevent IntegrityErrors when a user makes the same request
too rapidly and both sneak by the initial check.

Add capability for passing kwargs to AlertIssuer classes'
issue_alerts() method.  We take advantage of this in the update
alerts for checking for FAR changes and NSCAND state changes.

VOEvent IVORNs now use ivo://gwnet/LVC instead of
ivo://gwnet/gcn_sender. Also use the default superevent ID everywhere
in a VOEvent, including filename, even if it has a GW ID.

Add some information to VOEvents and rearrange some things:
  * Add EM Bright information (BNS, NSBH, BBH, Terrestrial) in a
    Group in the What section
  * Put ProbHasNS and ProbHasRemnant in a Group and rename to 'HasNS'
    and 'HasRemnant'

See https://git.ligo.org/lscsoft/gracedb/issues/77

We now use ValidateDestroyMixin rather than SafeDeleteMixin for
handling removal of events from a superevent.  Some additional
logic in other places was no longer needed.

All calls to event.graceid() -> event.graceid.

Improve log messages for signoff creation. Bugfix for handling
whether alerts are issued for "sub-functions" or not. Some
minor organizational changes.

New format for LVAlerts (XMPP alerts).  New class-based
functionality for issuing alerts for events and superevents.
See https://git.ligo.org/lscsoft/gracedb/issues/8.

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

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

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

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.

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.

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

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

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.

Using new create_versioned_file utility in create_log functions,
adding ability to provide queryset to get_superevent_by_date_id_or_404,
changing a few comments for autogenerated log messages

Added some utilities for handling signoffs for superevents,
including alerts, labels, logs, etc.

Adding API resource for creating and getting VOEvents from superevents.

Adding EMObservation creation and alerts for superevents.
Note that this required a change to gracedb-client.

First pass at implementing alerts for superevent creation,
update, annotation, labeling, etc.

Prevent users from removing a superevent's preferred event.
They will have to update the superevent to set a new preferred
event instead, then remove the event.

Created a web form and API resource for confirming a superevent as
a GW.  Also added and utilized permissions for allowing this action.