Remove padIds from the closed captions MongoDB collection subscription.
Users now have to fetch the padId from Meteor when needed. Meteor is
responsible for checking the user's access level and return the
proper id.
When managing Etherpad's pads, Meteor makes API calls to initiate the closed captions
and shared notes modules. The pad id was being mapped to a shorter id than the meeting
id because of a Etherpad lenght limitation.
Changed to something less guessable.
