Ensure that the session is logged out server side, not just client side.

Anytime a user clicks "sign out" we should be signing them out server side as well. This can be accomplished by setting the Users.current_session_id = null.
I found that the method User.logged_in_elsewhere doesn't need to check if the current_session_id is None. The current_session_ids in the cookie and db (redis or postgres) then the user should be forced to log in again.

app/main/views/sign_out.py

@@ -1,11 +1,12 @@
 from flask import redirect, session, url_for
-from flask_login import logout_user
+from flask_login import current_user, logout_user
 
 from app.main import main
 
 
 @main.route('/sign-out', methods=(['GET']))
 def sign_out():
+    current_user.sign_out()
     session.clear()
     logout_user()
     return redirect(url_for('main.index'))