New format of gallery information files

Comments for photos are now stored together with gallery information in a single
file. Its format is based on YAML. Names of image files are no longer required to
have names in form img-{num}.jpg and their ordering is specified in the info file.
Update ingconv script to new gallery format
Fix for situations when gallery date is not specified
imgconv: Make MQ images by default instead of LQ
imgconv: Remova also support for making ZIP archives
Remove repeated gallery initialization in index.php
index.inc.php: Use URLs from Gallery class
Clean up index.php and config.inc.php