Update District landing page for District-wide metadata

[thekaveman]

I wonder if we can make some small changes to the District landing page, e.g. /districts/7.

It seems more user friendly to present a map of ALL the stations in the district, rather than pre-selecting a single station. And/or a map that shows the full extent of the district. Pre-selecting a single station makes it more difficult for someone who is less familiar with the district to browse around.

I'm thinking of something like in the current PeMS, where landing on a District page, we see a nice overview map and some roll-up stats about the district: