Updated homepage header

[slifty]

The home page is going to now incorporate a new summary map which will give a snapshot of our dataset: https://projects.invisionapp.com/share/BQ7B3D4CM#/screens/158693517

• [x] Implement the design of the map using dummy data
• [x] Implement an API endpoint specifically for the map

[slifty]

@danletson is going to implement a "/api/v1/market_count" API endpoint in the plugin.