Openedx default changes OPEN-417

@singingwolfboy could you take a look?

I discussed these changes with Raph - he agreed that in the short term, we should make these changes to the "old" demo course (this is the course installed by default on Devstack).

In Q3, we're going to make & implement a plan to replace the default demo course on devstack with a sanitized / "Open edX" version of the new Demo Course. Part of that will be figuring out how to keep the open edx version mostly in sync with content changes/additions to the live edx.org demo course.


Tasks:
- [x] Change the FAQ to not reference a real textbook and promise it for free!!! maybe use supported browser question?
- [x] Get rid of "edX Community" tab in default demo course
- [x] Remove references to edX social media
- [x] Change "edX Demonstration Course" to "Open edX Demonstration Course"

I don't want to deal with changing the course triple to, eg, "openedx/...", because I think that might be a headache and we will be replacing this in a few months anyway.