Poi Search via overpass API

Search for arbitrary POIs via overpass API, display them in an overlay, and update when the map is moved/zoomed.

Needs https://github.com/mhanne/dspace-api-core/commit/956f93c732308416fd569164ac53ea8b18f91f34 and https://github.com/mhanne/dspace-ui-leaflet/commit/4f8c25dbb9dbb551b4272180193a29a507d5c1d4 to work.

Overpass also has an option to specify the search area as a polygon, so we could do incremental updates if we can calculate the subtraction and union of two polygons. Do we already have something that can do that as a dependency? I tried spatial_query and turf, but both don't seem to work with more complex polygons.

Also, add multiple basemaps to choose from; Sorry this got mixed in here, I can make a separate PR if you like.