foto: image album browser and control

Original foto.php controller and related assets copied from Lijtweg at
commit v2.5-24-ga5399c5d03 (2017-10-22).

Requires image files at data/ symlinked into albums at foto/ with prefixed
resolution attributes.  Depends on external javascript libraries [PhotoSwipe
and Flickr's Justified-Layout] at lib/ (changed from import/ at lijtweg)
installed by make rules.