federated cloud ID: add icons, fix HTML & layout, fix wording

apps/files_sharing/css/settings-personal.css

apps/files_sharing/img/gs-share.png

apps/files_sharing/img/social-diaspora.svg

apps/files_sharing/img/social-facebook.svg

apps/files_sharing/img/social-gnu.svg

apps/files_sharing/img/social-googleplus.svg

apps/files_sharing/img/social-owncloud.svg

apps/files_sharing/img/social-twitter.svg

apps/files_sharing/settings-personal.php

apps/files_sharing/templates/settings-personal.php