Transforming local stream to be a mirror, not real image from webcam

(easy CSS! :) )
WIP: small group discussions.  Backend for that works.

Now I need to change student's front-end behavior.
Groups discussions: calling someone doesn't work.
Ask for webcam access at the beginning of the session
Fixes #16 (thumbs/large video stream), next up: countdown

When I implement countdown, I'll be at feature parity with previous
version. Except that I'll have less bugs. :-)

A few things to take care of:
* someone wants to join the small groups call later on
* someone drops out of small group call
* all instructors drop out
* I think that's all…