add social media button for diaspora, twitter, facebook and google+

apps/files_sharing/css/3rdparty/gs-share/style.css

apps/files_sharing/css/settings-personal.css

apps/files_sharing/img/gs-share.png

apps/files_sharing/js/3rdparty/gs-share/gs-share.js

apps/files_sharing/js/settings-personal.js

apps/files_sharing/settings-personal.php

apps/files_sharing/templates/settings-personal.php