Jnf/scrubbering

__EDIT__: This is ready to go.

The basic idea is that admins can 'scrub' a response by creating a 'valid' (non-identifying, in almost every case) response to display at the normal blind level for the question. The admin also assigns a new blind level in which to display the original response. By doing this, we can preserve the original response (for future/final consideration) and the spirit of the infringing response (removing links, simplifying to yes/no, etc.).

Reviewers will see the scrubbed response (with a note to let them know it was scrubbed) until an Admin sets the Event's `blind_level` higher than the Scrub's blind level. At that point, reviewers will see both the scrubbed and original response together, so they have all the possible information.

I would really, really appreciate some code and concept review here. :heart: