Fixes #5: booking thumbnail not updated in members area when new default image is selected
 * added Ab4sBookingrManager::getDefaultImageUrl($UUID, $width, $height)
 * members area listings now use getDefaultImageUrl() instead of getOneImageUrl() to retrive the thumbnail because getOneImageUrl() always return the first image which may not be the default.
Fixes #5 : booking thumbnail not updated in members area when new default image is selected
fix regression introduced by commit 30f8c528080ea7c0f64a5749a1256c966b57b444 which causes the site to crash when no default image is set
fixes #5 : oops wrong variable name