[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?
these queries are getting heavy. I wonder if at some point we'll have to just use ES to feed the API.