feedbacks: don't send redundant dmails when user is banned or loses approver status.

When a user is banned, send them a "You have been banned" dmail instead of a "Your user record has
been updated" dmail.

When a user loses approver status due to inactivity, don't seen them a "Your user record has been
updated" dmail for the "Lost approver privileges" neutral feedback they receive.

test/unit/approver_pruner_test.rb

@@ -10,6 +10,8 @@ class ApproverPrunerTest < ActiveSupport::TestCase
       assert_equal([@approver.id], ApproverPruner.inactive_approvers.map(&:id))
       assert_nothing_raised { ApproverPruner.prune! }
       assert_equal(User::Levels::CONTRIBUTOR, @approver.reload.level)
+      assert_equal(1, @approver.dmails.received.count)
+      assert_equal("Approver inactivity", @approver.dmails.received.last.title)
     end
 
     should "not demote active approvers" do