Allow <img> tags in LTI2.0 feedback

@sefk 

This is to enable img tags in LTI feedback.
Interestingly, the examples for extending bleach this way in the [bleach docs][1] don't show unicode strings.  But the library itself requires unicode input, and spits out unicode, so what you're doing seems more correct.  So this seems fine.

  [1]: http://bleach.readthedocs.org/en/latest/clean.html#tag-whitelist