[Bug 1091781] Filter on question involved.

I forgot to add filtering by involved to the API. The easiest way to do this was to upgrade django-filter to 0.8, which was as easy as updating it in `requirements/default.txt`.

To test:

* Make sure to update your virtualenv with the new requirement. `./scripts/peep install -r requirements/default.txt --no-use-wheel`
* Run the tests
* Try something like `curl http://localhost:8080/api/2/question/?involved=cor-el`. For me this took a while, but only returned questions cor-el was involved in.

r?
I've been a little uneasy about the weight of the APIs too. I'm not sure if this will have a major impact on other parts of the site, or if it will only slow down API queries. I'd really like to get some metrics for this, but we won't really get that until we get the app done.

I think right now I'd rather work on getting it done, and then worry about making it fast later.