Provide list of suggested map tile servers

[lukasrad02]

The default map tile server serves satellite pictures without any annotations (e.g. city or street names). There are also other servers that provide map tiles with annotations, schematic imagery instead of satellite pictures and so on.

In the settings modal, there should be a list of some popular alternative tile servers and a description of what this server offers. When selecting a server, its correct URL should be entered in the map tile URL text box, but it should still be possible to enter custom URLs.