Community board district labels

[isaacbw]

Natalie: The community boards display on the map as “404” instead of “4" and “408” instead of “8" - I guess people just refer to them by the last number of their digits? I wonder if we could label them the way people refer to them. When I do a google search for community board maps, they just have the single numbers on them.

[allthesignals]

We should use slice here: https://docs.mapbox.com/mapbox-gl-js/style-spec/expressions/#slice

[allthesignals]

Pushed up a fix for this