Add field 'notify me about new routers in my area'

[andrenarchy]

In order to realize #31 and the ideas from freifunk-berlin/ca.berlin.freifunk.net/issues/27, we should add a field Notify me about new routers in my area. The field should probably be placed in the section router location and a radius should be specified and visualized in the displayed map.

Analogous to #31, this feature depends on major changes in the openwifimap/libremap.