Drop end to end signature. The impersonation attack vector is deemed out of
scope since insiders are already part of the meeting.

Signs and verifies frames. For now this is done on every frame.
This is not the long-term goal and there needs to be a better strategy.

in particular this allows sharing the ratcheting between the E2EEncryption
module and the Worker.

similar to what is explained here:
  https://tools.ietf.org/html/draft-omara-sframe-00#section-4.3.5.1
but we do it on authentication tag failures since it is not possible
to tell whether decrypt.

This the second stage in our E2EE journey.

Instead of using a single pre-shared passphrase for deriving the key used for
E2EE, we now establish a secure E2EE communication channel amongst peers.

This channel is implemented using libolm, using XMPP groupchat or JVB channels
as the transport.

Once the secure E2EE channel has been established each participant will generate
a random 32 byte key and exchange it over this channel.

Keys are rotated (well, just re-created at the moment) when a participant joins
or leaves.

Moves the logics which deal with JitsiConference events to a separate module.