Move the websocketUrl into the /api descriptor

This is part of https://github.com/hypothesis/product-backlog/issues/210#issuecomment-296169666

Currently the web service provides the `websocketUrl` setting to the client as a global setting in the `/app.html` page, but we'd like different annotation services to be able to provide different `websocketUrl`s.

In the future the client may connect to more than one annotation service at once. Which annotation service(s) the client connects to can be configured by the hosting page, when the client is embedded in a third-party page.

So the `/app.html` page isn't a workable place for the `websocketUrl` to live in the future.

The suggestion is to add `websocketUrl` to the JSON API descriptor at , then each annotation service can provide its own `websocketUrl`.

I think this'll require three pull requests: 1. Add `websocketUrl` to `/api`. 2. Once 1 is deployed, change the client to read `websocketUrl` from `/api`. 3. Once 2 is deployed, remove `websocketUrl` from `/app.html`.