merge -c2459 from release 1.7.2

New version 1.7.3 hard coded.

Stable release required modifications: don't show version and generation
time in footer, don't check the upgrade feed, don't die on sql errors.

git-svn-id: http://piwigo.org/svn/tags/release-1_7_3@2764 68402e56-0260-453c-a942-63ccdbb3a9ee