Notify on region change WiP (bug 888023)

This ended up being quite a rabbit hole.  I still need to add another message for paid -> free but that could be in another branch.

* I've split out disabled regions a bit to aid seeing what regions are generally disabled (e.g. always) vs regions that are disabled based on the price (Having them combined made doing different things based on XHR when the prices changed a problem.

* If you change prices and the checkboxes that are disabled are different from when the page was loaded a notice is shown.

There's still some clean-up to do on this but I'm keen to start getting some initial review comments on it.