Split street map into two layers with custom styles

[jdudley1123]

Context

The current map is quite hard to read because the pollution layer is over the top of the place names.

Changes in this PR

This PR replaces the stock map styles with two custom styles applied as two separate layers. The first shows the streets and water, and goes under the pollution layer. The second shows place names and goes on top of the pollution layer. This approach makes the map much more readable.

The appearance of these styles can be managed at https://cloud.maptiler.com/maps/

Trello: https://trello.com/c/dKiwJnEI/106-spike-to-understand-optimal-map-layering-styling-and-transparency

Screenshots of UI changes

Before

After

Next steps

• Run any necessary
  accessibility testing
  on this feature.

[patrickjfl]

Agree this is much more readable, although I guess at some point we will also need to do some work around duplication of labels in different layers...

[jdudley1123]

Agree this is much more readable, although I guess at some point we will also need to do some work around duplication of labels in different layers...

There's no duplication between the two map layers. There is a little bit of duplication where zone names are the same as labels on the map, but that's unavoidable.