Make 'deactivate group' option reversible

The 'deactivate group' option that appears under 'options' on a group's page ought to be a temporary deactivation that suspends all activity in the group, but can be reversed by a coordinator clicking a 'reactivate group' option in the same place. If there is a self-destruct mechanism, a 'delete group' option that requires intervention from the admins of the Loomio instance hosting the group to undo it, that ought to be under options > edit group settings > advanced settings. If there are 2 or more coordinators in the group, activating self-destruct ought to require approval from at least one other coordinator.