approver pruner: send weekly warning dmails to inactive approvers.

Send weekly warning dmails to approvers in danger of losing their
approver permissions. Don't send warnings if we're more than three weeks
away from demotion so that approvers aren't warned prematurely.

test/unit/approver_pruner_test.rb

@@ -27,5 +27,13 @@ class ApproverPrunerTest < ActiveSupport::TestCase
 
       assert_not_includes(ApproverPruner.inactive_approvers.map(&:id), @user.id)
     end
+
+    should "dmail inactive approvers" do
+      travel_to(Date.parse("2020-01-20")) do
+        ApproverPruner.dmail_inactive_approvers!
+      end
+
+      assert_equal("You will lose approval privileges soon", @approver.dmails.received.last.title)
+    end
   end
 end