Price tiers UI (bug 868179)

This branch adds the ui for the first take the improved ui for price tiers.
Here's the main changes:

* When the prices are updated the prices are fetched from the API with xhr and then the relevant regions that we have data for are updated and checkboxes.

One thing that is a byproduct of this change is that the regions is no longer shown when moving to the free tab (if a paid app) and vice-versa. This actually makes sense as it points more to the need to convert to a free or paid app first before changing/updating the other settings. The main reason for this is the need to not have two copies of the same region form fields in the page at the same time.

Optgroups based on price tier methods is coming in a future branch.