We should display a flash message to indicate whether or not subscription succeeded
If subscription is successful, we need to update the button text immediately. Currently you can only see that the text changed to "Unsubscribe to notes emails" if you reload the page.
Steps to reproduce:
This results in a 500 error from the server because we call the
subscriptions
endpoint with undefined values, e.g.:Clicking on the button should redirect to the login page for anonymous users.