Moves part of the XMPP events which are clearly
related to the PeerConnection to RTCEvents.

Fixes calling of leave in kick.

The manager will not forward the event if the participant does not
exist, as it is not expected to get the event for "undefined"
participant.

Our JSHint configuration is not extensive and we have excluded multiple
files from linting. Additionally, we have seen JSHint to be unable to
parse newer ECMAScript features such as the object spread operator
(proposal).

On the other hand, we have seen ESLint to beautifully work on React and
React Native source code in addition to ES2015 and later. Introduce
ESLint alongside JSHint as an intermediate step to eventually switching
from JSHint to ESLint.

As our source code does not fully follow even the JSHint rules we have,
it is very difficult to introduce ESLint with many rules. At the
beginning, introduce it with as little rules as possible. Morever,
ESLint is able to automatically fix certain rule offenders so once we
have ESLint merged we can incrementally add new rules while keeping our
commits' modification on as little subjects as possible.

When ICE disconnects, reconnects or fails the events will be logged
on both Callstats and "analytics".

The event is triggered when Jicofo sends 'session-terminate' and our
PeerConnection is closed.

Fix broken variable