Add search functionality to map component

[kylejb]

Depends on #3

What

Allow end-users to geosearch (e.g., search by address, postcode/zipcode or names of landmarks) from map.

How

Roll our own implementation or find a well maintained third party library that provides this feature (e.g., leaflet-geosearch).

[kylejb]

FYI - I'm going to poke around in https://github.com/clairBuoyant/react-leaflet-search (fork of react-leaflet-search) to experiment with bumping it to support latest versions of leaflet and react-leaflet.

If I can get it working, I'll try to get this merged up by the maintainer of this package; it would become another viable third party library, if successfully updated.