Add ability to check logged-in status by GETting /session

Adds a new route to `GET /session` which returns the following:
* `200 OK` and JSON response `{"status":"Logged In"}` if there is a valid session cookie sent to the server
* `401 Unauthorized` and JSON response `{"status":"unauthorized"}` if there is no valid session cookie sent to the server

This will let the app simply check whether a user is logged in so that it can decide to show the login screen or go straight into the app.