Isolate most Pad related functions and secure shared notes and closed
captions creation to only when there is a reachable Etherpad server
configured.

Since Meteor was split in multiple process and events started to be
filtered by instances, all Etherpad's Redis events were being discarded.

Etherpad has a Redis' publisher plugin that is unaware of BigBlueButton's
existence. All the communication between them is kept simple with minimal
of internal data exchange. The concept of distincts subscribers at Meteor's
side broke part of this simplicity and, now, Etherpad has to know which
instance must receive it's messages. To provide such information I decided
to include Meteor's instance as part of the pad's id. Should look like:

 - [instanceId]padId for the shared notes
 - [instanceId]padId_cc_(locale) for the closed captions

With those changes the pad id generation made at the recording scripts had to
be re-done because there is no instance id available. Pad id is now recorded at
akka-apps and queried while archiving the shared notes.

Associate pads with meetings so session validation is restricted to the
meeting's valid session tokens.

Meteor will dispatch new redis events on shared notes and closed captions
pads creation. This event will go through apps and reach web to populate
a new meeting's pad collection that contains all valid pad id's for that
session. Nginx will use this collection to check if the user's session token
belongs to the pad's authorized users.

Besides these modifications, an extra change will be needed at notes.nginx.
Location /pad/p/ needs to change it's auth_request:

from /bigbluebutton/connection/checkAuthorization;
to /bigbluebutton/connection/validatePad;

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.