Add custom URL imagery choices

[jidanni]

Sooner or later somebody's going to (be in a hurry and) want to use a different imagery than the ones offered, so there should be at least one custom URL1, custom URL2 etc. templates.

[deevroman]

And this applies not only to satellite images, but also to the main map. For example, so that it is possible to add maps with z20 https://osm.cquest.org/dev/#20/44.13530/4.81051