Fix gallery app for non connected user

This patch fix the sso too strict about the gallery app. Indeed non connected user (share link) could'nt see the diaporama.
Skip apps/ seems to be the best way to be sure that all functionnality of owncloud work. Normally base app are checked by owncloud team. About 3rd party apps, it's different their quality isn't guaranted, but if the yunohost administrator took the time to copy the file across an sftp and activate the app, I think yunohost should allow the request from these apps.