Use em-dash to separate name from abbreviation

This is a clearer separator than a pipe in the context of the
organisations membership editing box.
Add flag disabling editing of user memberships of organisations

The 'View Organisations' page is still visible to admins, but the
user edit form doesn't allow editing of organisation membership.
Remove closed_at datetime from organisations

Not all closed organisations in Whitehall have a closed_at datetime
set (2 out of 4 do have it) so it's not reliable for finding closed
organisations. If we do want to provide different behaviour in future
for closed organisations or those which are exempt from transitioning
to GOV.UK, govuk_status from Whitehall is the most accurate available
source for that information.