If a tag's name has some "uncommon" characters in it, like
parentheses (or probably others), it will cause errors for the
javascript which renders the event pages, since it tries to use
the tag name in the element id for the delete button.  We now require
that tags only use 0-9a-zA-Z_\- in their names. May have to go through
and delete some tags which use them.

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.

Changes to the eventlog API caused issues for calls to that resource
from within the javascript which populates the event log on the web
view for an event, since the issuer is no longer a dict containing
'username' and 'display_name' keys, but rather just a string
corresponding to the username.

Shortcuts modules were added for the events and superevents apps.
Currently, they just contain 'is_event()' and 'is_superevent()'
methods.

Splitting up much of the alerts functionality and
improving/generalizing it.

Some imports in superevents/serializers.py were moved to
function-level.  This is because several functions from
superevents/utils.py are used in the serializers for
deserialization (object creation), but the serializers are also
used in the alerts app to serialize objects to dicts.  This created
a circular import issue.

We also fix some of the reverses so that they work without a
request object being present.

Activate Django sites app and create a migration which adds the
LIGO.ORG domain name of the GraceDB instance as a Site. This will
be used to build absolute URIs without needed a request object.

Instantiating a new alerts app and moving the alert functionality from
events/alert.py to alerts/old_alert.py. This commit forms a starting
point where the old alert functionality is still in place and we can
rework the alert infrastructure to be more modularized, flexible,
and adapted to superevents.

Now using get_superevent_by_date_id_or_404 to retrieve the superevent
for log creation via the web interface.

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

Remove commented out code blocks for file handling in
superevents/forms.py.