goto existing principal link directly if the principal is selected in "ADD USER" or "ADD GROUP"

usually admins can click a principal link directly to edit its rights, but if
one missed the link and used the input below "ADD USER" or "ADD GROUP"
instead, then it's treated like a principal without any right, which is kinda
misleading. this commit fixes it by automatically switching to existing links.
Merge branch '4.0/add-existing-principal-in-admin-rights-ui' into 4.0-trunk