Refactor Country Model

Due to the treatment of the PK in projects.models.Country, and the inconsistencies with mapping Datamap.js country data to django-countries data, this PR:

* resets migrations
* adds a management command to update Country based on Datamap JSON
* Refactors Country model
* adds fixture for Country
* revisions to the homepage map example
  * change to world map
  * populate datapoints with actualy Project(s) data

There are still some issues to sort out with the map based on finalizing the home page design, but those can be resolved in other tickets.

**This PR will require you to drop and recreate your DB**