Remove usage of .keys(), .has_key(), and .iteritems(), and
account for the fact that .values() produces an iterator in Python 3.

Previously, core.vfile.VersionedFile inherited from file,
which is problematic for several reasons, but primarily because
file doesn't exist in Python 3. So we rework it to just be a
normal class with a file attribute, and add/change certain
methods accordingly.

Import version of urlencode which is packaged with Django in order
to facilitate Python 2-3 compatibility.

we need to dance between str (unicode) and bytes to talk to hashlib and StringIO at the same time

only required on python2, but this code will break on python3 anyway

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