add matt stiles' tree data repository for LA county + cities

[emillipede]

We've had a number of requests to expand the map to other parts of LA County (eg LA City, Alhambra, unincorporated LA County).

Matt Stiles, a reporter at the LA Times, has collected data about street trees many LA County cities and put them in a github repository. WeHo and a few other cities publish public datasets, but he's included them in his repo, which seems like the best resource for us.

Let's explore how we can add these files to the current data pipeline and map (minus his Santa Monica dataset, which is less up to date than the csv from Santa Monica's open data portal).

• add "city" field to trees json (or database)

These datasets aren't regularly updated (either by Matt or the municipalities).

in progress branch: feature/la-data