Use the "apiRoutes" service to get the URLs of the `/oauth/authorize`
and `/oauth/revoke` endpoints from `/api/links` instead of from
"oauthAuthorizeUrl" and "oauthRevokeUrl" in app settings.

This makes the client's behavior more consistent in terms of getting all
links to pages within the service from the `/api/links` route.

It also paves the way to enabling the client to use multiple annotation
services, each of which is defined by a single entrypoint (the `/api`
route) from which all other API routes and links into the service are
obtained.