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.
Move Candy.js files to new vendor/ directory
Remove additional jQuery include

jQuery already available across the whole site at the top level, so
no need to include it again specifically for the chat widget.
Test generation of chat settings

Ensure that the chat connection settings are generated properly for
the template context.
Add setting to enable/disable chat site-wide

To protect against chat blowing things up, we include a way to
enable and disable it on an entire site-wide basis.
Use chat settings when deciding whether to render

Only render the chat widget if both the site has enabled it in the
`MITX_FEATURES` and if the course has enabled it. In addition, fail
gracefully with a log warning if the `JABBER_DOMAIN` is not set, and
do not try to render the widget. However, do go ahead and render the
rest of the courseware.