Peter Tran did a lot of preparatory work for showing the scroll

position of your peers in a TogetherJS session. This work was

landed in a project branch, awaiting some clean-up once the term

was over.
This is the clean-up I had in mind. I've fixed the following:
* Scroll position indicators now have a high z-index to avoid being

  hidden behind other elements.

* If no hub URL has been specified, properly falls back to the Mozilla

  TogetherJS hub.

* Moves the ReviewTogether JS code into a closure to avoid polluting the

  global namespace.

* If a peer's colour changes during a session, this is now manifested

  for everybody in the session without requiring a page reload.

* Switched a lot of double-quotes to single-quotes.

* Leaving a TogetherJS session removes all scroll position indicators.

* When somebody leaves a session, their scroll position indicator is

  removed.

Installed the extension locally. Then, viewed a long diff in a local review request.

Clicked on the 'Chat' link in the header to open the TogetherJS interface. Got the

sharing link from TogetherJS.
Then, in a second browser, went to the sharing link. I was able to view the scroll

position for both browser sessions on the long diff.