Make get_user_session break properly

This change will intentionally throw a RecordNotFound-error to prevent a
nil-error later on. The intended behaviour is to log the user out and
redirect to login, but this is currently impossible because helpers
can't redirect in all cases.

The authentication helpers should probably be moved to a concern, but I
can't be bothered enough, so it remains fixme.

app/helpers/authentication_helper.rb

 
   # Determine whether the user is logged in, and if so, disable the Session, then flush session cookies.
   def log_out
-    if is_logged_in?
+    if is_logged_in? and @user_session
       get_user_session
 
       @user_session.update!(active: false)
     if @user_session
       @user_session
     else
-      @user_session ||= Session.find_by(
-        id: cookies.signed.permanent[:session_id]
+      @user_session ||= Session.find(
+        cookies.signed.permanent[:session_id]
       )
     end
+
+    # Edge case if a session no longer exists in the database
+    if not @user_session
+      log_out
+      redirect_to login_path # FIXME!
+    end
   end
 
   def current_user