Jbau/coe sanitize with bleach

@VikParuchuri @brianhw 

Use the bleach library (https://github.com/jsocol/bleach) instead of lxml.html.clean to do html sanitization.
We found that the previous library didn't handle inlined `<` well, so if a student wrote something like `3 < 5` in the submission, the part of the submission after `<` would be entirely missing.  bleach appears to be much more tolerant, since it's based on html5lib.

@jtauber License is here: https://github.com/jsocol/bleach/blob/master/LICENSE