Will actively terminate P2P session by the responder (not moderator) in
order to shutdown P2P in case of eventual initiator's crash. Otherwise
the responder will stay in P2P for too long (until P2P ICE fails).

Prevents from printing Jingle 'session-terminate' error response in case
both responder and initiator terminates their sessions simultaneously
(gracefully). In that case 'item-not-found' error is returned by each
party, because the session is removed immediately from the memory on
termination (see strophe.jingle.js).

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

Introduce JingleSessionState constants. Make the invalid JingleSession
rejection an explicit action taken by the JitsiConference.

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.