URL algebra

The ADMIN_URL variable in linolib.js didn’t correctly handle the case of an empty lino.Lino.admin_prefix.

This was because I had once again tried to reproduce Django’s rule for MEDIA_URL which must end with a slash if set to a non-empty value.

After fiddling with different constellations I now gave up. I just cannot figure out why the hell they have such a rule. IMHO the rule should be easy because that’s a common configuration option, but not at the price of performance. Added a note about the fidderence in the docstring of lino.Lino.admin_prefix.