Reorganized directory structure. And implemented some kind of API.

PHP scripts are now divided up into subdirectories, one for serving pages
and one for submit scripts and includes. To hide these changes, clean URLs
are now provided for all actions.
CSS cosmetics and other corrections
Introduced editorial comments
Introduced a "public" flag for problems and solutions.
Show non-public problems and solutions to editors only

Also distinguish them by border style and opacity from public ones.
Add cleanup script
Generate password for newly created user and send it
Replaced close button image by Font-Awesome button