Superevent ID assertion no longer holds

Superevents IDs no longer have to start with "S".

Traceback (most recent call last):
  File "/home/emfollow/.local/lib/python3.6/site-packages/celery/app/trace.py", line 382, in trace_task
    R = retval = fun(*args, **kwargs)
  File "/home/emfollow/.local/lib/python3.6/site-packages/celery/app/trace.py", line 641, in __protected_call__
    return self.run(*args, **kwargs)
  File "/home/emfollow/.local/lib/python3.6/site-packages/celery/app/base.py", line 467, in run
    return task._orig_run(*args, **kwargs)
  File "/home/emfollow/.local/lib/python3.6/site-packages/gwcelery/tasks/circulars.py", line 10, in create_circular
    return ligo.followup_advocate.compose(graceid, client=gracedb.client)
  File "/home/emfollow/.local/lib/python3.6/site-packages/ligo/followup_advocate/__init__.py", line 34, in compose
    assert gracedb_id.startswith('S')
AssertionError