Add a map-pin at this address in this map view

TODO: write docs

On clicking on the pin, navigate to the electors at the address. This way of doing it adds an antry in the browser location history, so back links work.

Remove all pins from this map view. Side-effecty; liable to be problematic.

select the name of a suitable pin image for this address

Refresh the map pins on this map. Side-effecty; liable to be problematic.