Simple port of Class2Go's chat feature

Embed a chat widget (much like the calculator widget) into the
courseware. To use, you must point it at an ejabberd box,
configured as `JABBER_DOMAIN` in the settings.

This is a [reissue](https://github.com/edx/edx-platform-pre-clean/pull/2081) from the old repo.

Folks involved: @cpennington, @jrbl, and @dianakhuang