The Default Bit Rate dropdown on the Transcoding create/edit forms was fed
BITRATE_CHOICES, which starts at 32. There was no way to pick 0, and the
SelectInput was not resettable, so an admin could neither create nor restore a
transcoding with no default bit rate, such as the default FLAC one (seeded with
0 in consts.DefaultTranscodings). Editing that row also rendered a blank
dropdown, since its stored value matched no choice.
Adds TRANSCODING_BITRATE_CHOICES, which prepends a 0 entry labelled 'None' to
the shared list. The forms use it as SelectInput choices, and the list and
read-only show view render it through SelectField, so all four screens resolve
the label from the same array and cannot drift. The shared BITRATE_CHOICES is
left untouched, because 0 is not a meaningful option for the player Max. Bit
Rate or the share dialog.
Reported in discussion #6107, where a user had deleted the default
transcodings and could not recreate the FLAC one.
