Limit jobs shown by site

This doesn't handle a ton of edge cases and allows for filtering on the current site only. It will break My.jobs postajob pretty much entirely at this point, which I'm trying to find a way around. I'm also trying to think up all the edge cases. At a little over 900 lines, it's time for someone to at least start reviewing it, though.

All tests pass.
are we deprecating the use of `factory_boy`?