Add support checking the permissions of symlink targets to the storage directory helper

Fixes: #1597 (closed) Fixes: https://gitlab.com/gitlab-org/gitlab-ce/issues/22878 Fixes: https://gitlab.com/gitlab-org/gitlab-ce/issues/22907 Fixes: https://gitlab.com/gitlab-org/gitlab-ce/issues/22983

Note that while the repositories storage directories cannot be symlinks. (Not exactly sure why, but that's what our docs say) The other storage directories seem to have been working as symlinks previously.