Implementation for WebRTC based Video Calls

[HarshNarayanJha]

This PR implements simple WebRTC based Video Calls. This adds the required server events, and handling.

Newly added server events are:

INCOMING_CALL = 'incoming_call',
CALL_DECLINED = 'call_declined',
SET_OFFER = 'set_offer',
SET_ANSWER = 'set_answer',
SET_CANDIDATE = 'set_candidate',

All handling is done by src/handlers/webrtc.ts

Related Frontend PR: Suyious/purrr.frontend#47