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.
Does this need the check? Will `annotation['user'] = ''` not be sufficient? Or do we want to only clear the author if it was originally present?