Concept XBlock prototype

@nedbat @jarv Re-issuing PR due to previous force push on edx-platform. 

This is intended for a limited-scale proof of concept for the physics course. It should not be used elsewhere. 

In particular:
* It points to my machine.
* It does not handle Studio well at all.
* The theme doesn't match the LMS, and it is a bit bigger than our viewing hole. 
* It is very slow in the LMS -- there's a minor war between it and our Mathjax code. That, combined with an unnecessarily large numbers of AJAX calls, makes the user experience not great.

All of these are easily fixable, but the goals are: 

1. To let users know that they are being listened to. This is a 3 week course. In week 2, they asked for concept tagging, so we'd like to get that in quickly. 
2. To get feedback from a few (less than two dozen) users in a live course to see if this is a logical way to structure things before investing more dev effort, or if there is a better approach.