Bug 1030996 - Add in tool notifications

This is a draft for feedback
You asked about heavy vs light middlewares. I don't think there's a general opinion on this, but I don't think it really matters if the middleware is lightweight or not. `check_notification` here isn't really going to be used anywhere else, and isn't really usable as a generic utility; returning `False` or a URL to redirect to is a pretty specific use here.

I'd move the code back into this middleware. As it is, anyone trying to figure out what this middleware does is just going immediately to utils anyway. :P