Add field 'message to the community'

[andrenarchy]

The idea popped up in freifunk-berlin/ca.berlin.freifunk.net/issues/28 and freifunk-berlin/ca.berlin.freifunk.net/issues/27.

A field Message to the community could be placed in the section contact details. If another user subscribed to notifications about new routers in the area, then this message will be shown (together with further information about the router).

Note: the actual realization of this feature depends on major changes to the openwifimap/libremap service.

[sarumpaet]

We can also implement that backend functionality on util.berlin.freifunk.net - which I think is a better place than directly in the map (backend).