[bug 1062436] Add flags app and Flag model

r?
I decided to go for "very simple" for now. I added notes to all the bugs talking about how this is ultra-simple and just for the prototype.

So the code that runs spicedham will flag with "spam" and "abuse". Then we'll have buttons in the analyzer view I'll write that will add "wrong-spam" and "wrong-abuse" which covers the false positive and false negative cases and lets us do reports like "what are all the responses that aren't spam, but were marked as spam?"