bug 1042820 - moor tidy of apps/amo

Not all of it, by a long way, but the bits I could clearly see were used or unused.  Also I figured I better push the progress so far while its still (semi-) understandable.
This is generally useful. I'd vote we put this in `mkt/site/fields.py` as it's not just a devhub thing even if that's the only place it might be used now.