Bug 10411: add cache control headers for static resources (Debian package)

Bug 10411: add cache control headers for static resources (Debian package)

This adds caching directives to the static resources so that the browser
doesn't ask about them all the time.

By default, Debian apache doesn't have the required module (mod_expires)
enabled, and so this patch will have no effect. In order to enable this
function, run:

a2enmod expires

and restart apache.

Test Plan:
* Using firebug or equivalent, load a page.
* Note that none of the images, css, or js files have a Cache-Control or
  Expires header set.
* Add this patch, redeploy the package with it (or overwrite the
  apache-share.conf file) enable the 'expires' module, restart apache.
* Force-reload the page
* Note that the images, css, js now have Cache-Control and Expires set
  for 12 hours into the future.

Signed-off-by: Galen Charlton <gmc@esilibrary.com>
Signed-off-by: Jonathan Druart <jonathan.druart@biblibre.com>
Signed-off-by: Galen Charlton <gmc@esilibrary.com>