Improve usability of organisation select on batch_invitations/new

This makes it easier to select the organisation for a batch of users
by excluding closed organisations and sorting the remaining ones
alphabetically. We did something similar in #2426 and #2417 - there's
enough subtle differences in those two select boxes (indicating a
current organisation and adding a "None" option respectively) that I
haven't attempted to extract the duplication. I think the new scopes
are self-explanatory enough that I'm happy to keep them inline in this
view.

app/views/batch_invitations/new.html.erb

@@ -73,7 +73,7 @@
           id: "batch_invitation_organisation_id",
           name: "batch_invitation[organisation_id]",
           label: "Organisation",
-          options: policy_scope(Organisation).map { |organisation| { text: organisation.name_with_abbreviation, value: organisation.id } }
+          options: policy_scope(Organisation).not_closed.order(:name).map { |organisation| { text: organisation.name_with_abbreviation, value: organisation.id } }
           } %>
 
     <% end %>