Implement is_admin and is_curator functionality

[adkinsrs]

Awesome! Does this included respecting the guser.is_admin and guser.is_curator settings yet? We need to elucidate out what those mean still.

Originally posted by @jorvis in #671

This is the only bulletpoint left from #671. Maybe it's been a while, but I think it would be good to list when these two user statuses should provide to the user, at least for the dataset curator page.