Initial commit of cms and lms python, html, css and js changes

@jrbl & @stvstnfrd - making it possible to add arbitrary HTMl to course_about sidebar
Use a named placeholder to provide context for translators.

Also, this variable name is misleading; the value contains html, _not_
text.

```pythong
html_sidebar = _("Custom sidebar content for {link} (formatted in HTML)").format(link=anchor)
```