Use the human-friendly interval syntax for Apache caching

Use the human-friendly interval syntax for Apache caching

Apache mod_expires has supported a human-readable syntax for defining cache
expiry directives for a long time. In this commit we simply convert the
existing defaults from the "Anumber_of_seconds" syntax to the equivalent
English syntax documented at
http://httpd.apache.org/docs/2.0/mod/mod_expires.html

Whether CSS _should_ have a cache lifetime of 50 minutes by
default is left to the subject of future commits :)

git-svn-id: svn://svn.open-ils.org/ILS/trunk@19825 dcc99617-32d9-48b4-a31d-7c20da2025e4