Make separate administrator namespace (wincent.dev, 13a0aec)

In general I prefer in-place editing where possible but there are some cases where it is useful to have an administrator-only view of a model. For example, consider the Tag model, where it is nice to have a read-only "cloud" view for normal users, while for admin users we can provide a straightforward table view with in-place editor fields and sortable table columns.

Not sure why, but at some point my "admin_only" and "logged_in_only" helpers broke, so I've simplified them by removing the "simple_concat" method and now they're working again.

Signed-off-by: Greg Hurrell <greg@hurrell.net>