consume region stuff from marketplace constants (bug 1099282)

The authoritative list of regions is in marketplace-constants.mpconstants.countries

- A JS module is built and distributed in bower: ```require('regions').REGION_CHOICES_SLUG```
- A CSS module is built that exposes a list of regions as a variable and has rules for country flag background images 
- Only supported regions have their country flag images distributed in Bower, we just copy them all in, and gitignore what we had before

Note: I removed the 'None' key from REGION_CHOICES_SLUG, I wonder if that will break anything.

Also note: I do have plans to require JS modules straight from bower_components.