Add explicit 'Locate me' control to map

[mvl22]

It is helpful if the user could have a 'Locate me' button to do an explicit geolocation on request.

One example is use-case is perhaps having moved away from their current geolocation and wishing to relocate. Or, if on a bus and adding a location, it is helpful not to have to do a full page reload.

https://github.com/domoritz/leaflet-locatecontrol