[Bug 1089748] API: Filter by question metadata

Sorry about the extra commits. I'll rebase those away later. The interesting on is the last commit.

This adds another heavy filter to the questions api. It adds the ability to filter questions by the metadata they have. I talked with espressive about it a bit, and we agreed that a JSON string that has the keys and values to filter on in the querystring made sense. You might have a query like `/api/2/question/?metadata={"os":"Linux","category":"troubleshooting"}&involved=mythmon`, which would filter for Linux, the "troubleshooting" category, and things that I have asked or answered.

r?