[Bug 1074959] Add metadata API.

This isn't as nice as I wanted. The metadata field of questions is a list of objects in the API. Semantically it should be a dict, but I couldn't get DRF to play nice with writing in that format. There is some support for writing directly to the object with POST and PATCH, but it's kind of derpy, so I also added endpoints for `set_metadata` and `delete_metadata` that work intuitively for single items.

r?