Feature: Users adding layers to RouteBuilder/LabelMap

[misaugstad]

Brief description of problem/feature

From @yeisenberg on Slack:

how difficult would it be to allow for uploading contextual shapefiles? or for @misaugstad to upload on the backend shapefile or link to GeoJSON? asking because had a conversation today about mapping the routes to voting locations.? If we could display the voting locations, it would make it easy to then draw the route from a nearest bus stop or intersection to that location

And then from @jonfroehlich

Oh, that's such a neat idea! Basically, you want us to add in another layer to show key locations in things like LabelMap and the RouteBuilder tool?

I think that letting users add a layer to our maps should be doable, at least if we're doing this entirely on the front end so that we don't have to store anything on the server. Mapbox lets you add a bunch of layers, so we'd basically be adding a UI component that calls those underlying Mapbox functions. It's possible that there are even existing plugins that we can just use??

If we have to do the implementation ourselves, then I think that the factors that increase complexity are:

• Adding multiple file type options: GeoJSON, Shapefile, etc
• Allowing different types of layers: Points, lines, polygons
• Allowing an arbitrary number of layers instead of just one