make keyboard / screen reader map navigation better

[techieshark]

Sighted users are able to use their eyes and a mouse to quickly see which schools are nearby.

However, the source order of the map markers is not based on distance from the user, so if you tab through the markers you'll be jumping around to schools in seemingly random positions in the map's view bounds.

We could perhaps instead:

1. add map markers based on distance from user, so that when you use the keyboard to TAB through the markers, you'll be visiting the closest ones first.
2. Include some textual information in the marker indicating how far the school is from your given address.

See also: WCAG 2 H4: Creating a logical tab order through links, form controls, and objects

This might just be a bit of SQL to get Carto to order things correctly, but I'll leave it for later to look into that.