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:

* Prices (Tiers) select is moved to a normal set of choices which is generated so we can use nested tuples to get optgroups.
* The logic to work out the groups can be updated in the future to get rid of the decimal comparisons when we have data on the price method (carrier/card+carrier) etc.
* 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.

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.