merge r22402 from branch 2.5 to trunk

bug 2894 fixed: make sure all users cache is reset when an admin changes the
album thumbnail.



git-svn-id: http://piwigo.org/svn/trunk@22403 68402e56-0260-453c-a942-63ccdbb3a9ee
[trunk_public] Update mn_MN, thanks to : Ulzii

git-svn-id: http://piwigo.org/svn/trunk@22408 68402e56-0260-453c-a942-63ccdbb3a9ee