Better handling of case where a certificate already exists, but is
assigned to a different user.

Display the "RETRACTED" part first.

We now try to get the skymap filename from the latest non-retraction
VOEven and find a related PNG file. If that fails, try to find
bayestar.png. And if that fails, don't show a skymap.

Users in this group have permission to "T90", or update certain
parameters of GrbEvents in GraceDB

Add a new API endpoint for updating certain GRB event parameters:
ra, dec, error_radius, t90, redshift, designation. Only users with
specific permissions can use it.

Found a few more edge cases when applying this in production,
so I'm patching this migration retroactively.

Trying to remove a label that was not associated with an event
gave a 400 HTTP response. Fixed to be 404. It was already correct
for superevents.

Things like 's190418BC' will work now (should be 'S190418bc')