button must be visible but disabled if nothing to save

git-svn-id: https://svn.apache.org/repos/asf/archiva/trunk@1470480 13f79535-47bb-0310-9956-ffa450edef68
fix issues downloading merged index path (thanks to unit tests) and add an other with non default mergedindex path

git-svn-id: https://svn.apache.org/repos/asf/archiva/trunk@1470481 13f79535-47bb-0310-9956-ffa450edef68