Remove the KoboUserKey-based Authentication.

Instead, the user generates the api_endpoint url to set on their device
by visiting http://.../kobo_auth/generate_auth_token.
The generated url will contain a RemoteAuthorizationToken that will be
included on all subsequent requests from the device to the kobo/
endpoints. (In contrast, the device is authenticated using a session cookie on
requests to the download endpoint).

Also use Flask.url_for to generate download urls.

cps.py

@@ -42,6 +42,7 @@ from cps.admin import admi
 from cps.gdrive import gdrive
 from cps.editbooks import editbook
 from cps.kobo import kobo
+from cps.kobo_auth import kobo_auth
 
 try:
     from cps.oauth_bb import oauth
@@ -61,6 +62,7 @@ def main():
     app.register_blueprint(gdrive)
     app.register_blueprint(editbook)
     app.register_blueprint(kobo)
+    app.register_blueprint(kobo_auth)
     if oauth_available:
         app.register_blueprint(oauth)
     success = web_server.start()