Add Course About Page sidebar text area

- Add text area in Studio 'Schedule & Details' page
- Store text area content in mongo
- If text area in Studio has content, display sidebar section

@jrbl & @stvstnfrd - making it possible to add arbitrary HTML to course_about sidebar
You're right; it is more readable. It's also an unrelated change.

As a general rule, we should try to make only the minimal changeset
necessary for features and split cosmetic updates into a separate
PR/commit.  Especially since this is _not_ going to land upstream
(yet?), we should try to keep the diff between `origin..upstream` as
small as possible.

Also, I've already rewritten this whole block and when I PR it to
upstream later today, it'll just blow this line away anyway :P