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

The OpenLayers JS/CSS download URL is dead. Update it to make the map
work again:

- Update from OpenLayers 5.2.0 to 5.3.0
- Switch from the obsolete rawgit.com URL to jsdelivr.net (rawgit.com
  was only redirecting to jsdelivr.net for the last few years anyways)
- Set a fixed commit in the URL, so the URL doesn't become outdated again

This removes PKG_VERSION and PKG_RELEASE from most Makefiles, as the
value was never useful for Gluon packages; instead, PKG_VERSION is set
to 1 in gluon.mk.

It also removes two other weird definitions:

- gluon-iptables-clamp-mss-to-pmtu replicating the old PKG_VERSION logic
  from gluon-core, but without the fixed PKG_BUILD_DIR to prevent
  unnessary rebuilds
- gluon-hoodselector set GLUON_VERSION=3

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.

Signed-off-by: Jan-Tarek Butt <tarek@ring0.de>