Adding Mapbox vector tile basemaps

Great seeing the updates and progress here @zestyping @langstonsmith @Marena.

@Marena Agreed with the proposal for having a single account for the official ODK Collect app and then giving dev's options for fork's or working with a fork of ODK Collect. I don't see any issues with this approach at the moment.

@zestyping Following up to your question about @paul.uithol's comment. Yes, I think that's in line with what he was talking about. Being able to configure based on different online or offline options is something we'd be looking for. Some of the other items, like editing, sound like are beyond the scope of this thread and are starting to get towards this new thread: Load existing data points in to a geopoint widget in Collect