Make TinyMCE more tolerant about HTML elements

STUD-1565

This change improves TinyMCE's handling of some HTML elements (e.g. span tags, math tags etc) but doesn't completely solve the issues our users are reporting: 

http://help.edge.edx.org/discussions/problems/4718-help-me-understand-what-studio-strips-from-my-code

In particular, TinyMCE will still rewrite the user's HTML, losing whitespace, comments etc. It also loses <link> tags which strictly speaking aren't allowed outside the head tag, but for some reason seems to be used as a workaround in most browsers. I tried all of the TinyMCE options I could find, but I think I've hit the limit now.

@cahrens Let me know whether you think this is a net win, or whether it doesn't get us far enough. Either way, we still need your changes, I believe.

@nasthagiri Could you also review this when you have a chance.