When chat is enabled, sync user accounts with the ejabberd server
by directly writing into its MySQL database. This is ugly in the
sense that ejabberd just does a simple string comparison when
authenticating a password against a username, so we have to send
passwords in the clear in the templates. In a future commit, I'll
implement a password rotation mechanism to make this slightly more
robust: currently, everyone's password is just a dummy string.

To enable this, you'll have to add a 'jabber' key into your
DATABASES dict in the settings, and point at an actual ejabberd's
MySQL instance. The current plan is to just point at one that we've
got set up, since running ejabberd locally (not to mention on
MySQL instead of Mnesia) is non-trivial.

Rather than checking to see if the JABBER_DOMAIN is not set and
continuing on, bail with an ImproperlyConfigured exception as soon
as we notice its absence.

Per
https://edx-wiki.atlassian.net/wiki/pages/viewpage.action?spaceKey=ENG&title=I18n+coding+guidelines
ensure that chat messages are properly internationalized.

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.

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

Ensure that the chat connection settings are generated properly for
the template context.

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

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.

Fix typo and an unclear description.

[STUD-256] Seems like on chrome when cancel is hit in the file chooser, the 'change...

Fix broken lettuce tests

Feature/dhm/timeconv

Update lettuce test for problems with new css for show and hide answers.

Seems like on chrome when cancel is hit in the file chooser, the 'changed' event is fired and startUpload() was being called. We need to check to see if the upload file is blank and - if so - just return immediately out of the function

A number of important accessibility changes

Reviewed by @brianhw https://rbcommons.com/s/edx/r/17/

lms - adds back in missing/previously added .sr screen reader-focused styling and references in place of the .visuallyhidden application

adds in status for overall problem status indicator and replicates needed correctness django logic to show individual choice status