MMV: Allow <sub> and <sup> in captions
As we already allow visual markup in captions, I see no reason to not
whitelist sup and sub. These elements are even allowed in the
displaytitle whitelist for articles, so seems appropriate to have them
for media captions as well.
Bug: T186844
Change-Id: I4339ee85d139301770a4e15650c9dc0d64c136cd