changes ln from third to first form (see man ln)

Allow replacing the default OSM layer with a custom XYZ layer in
site.conf.

The new code is shorter and uses more readable variable names. It does not
depend on specifically named input fields anymore (allowing to use multiple
maps on the same page), and only uses well-defined interfaces to trigger
revalidation of input fields.

The Map model class allows to add OSM maps to gluon-web forms.