We're going to use tags. From the application standpoint, a question is only in one category. So we need to be able to delete previous categories or have the API do it for us.
This will probably work similar to (though not exactly like) the metadata/settings APIs. /api/2/question/<id>/addtag and /api/2/question/<id>/removetag, since that will be simplest to implement. There will also be a new "tags" field on the question APIs.
PR: https://github.com/mozilla/kitsune/pull/2360 Merged: https://github.com/mozilla/kitsune/commit/2b363af5192d214802a78d91ef0fcb3de718df1f This is deployed to stage.