Add optional Site Selector

[maurerle]

The site selector looks like an interesting feature available at 3laendereck.
Other domain-based maps might be interested in this as well:

https://map.freifunk-3laendereck.net/

The source of the outdated fork is here:
https://github.com/srauscher/meshviewer

@srauscher - are you interested in getting this here? :)