Refactor API code to use annotator-store flaskless

Everything seems to work: reading, creating, updating, deleting, searching, permissions seem to work but need better testing, and in general more tests and thorough code review would be good.

To keep a consistent style and make refactoring easier, I decided to use traversal with a `@view_defaults` class everywhere, using the defaults class to specify the `context` predicate, so as to group the views within that. For the same reasons I do not use view names but use a resource for each location (I explictly state `name=''` for clarity, though that could be removed). I think this style works out nicely.

I put some questions in the comments, just search for the question marks.

The dependency on the next (flaskless) annotator-store version has yet to be added.
Alright, fine. If we need to use the `auth.User` object for now, you can do it. But you should use `request.client.client_id` instead of `get_consumer` here, otherwise you'll be hitting the database a second time unnecessarily. Where `Token#validate_request` is setting `request.user` you should also set `request.client`. When the authentication policy calls `request.verify_request()` that delegates to the token handler's `validate_request` which calls `get_consumer()` already.