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.

app/logical/approver_pruner.rb

@@ -30,4 +30,19 @@ module ApproverPruner
       end
     end
   end
+
+  def dmail_inactive_approvers!
+    days_until_next_month = (Date.current.next_month.beginning_of_month - Date.current).to_i
+    return unless days_until_next_month <= 21
+
+    inactive_approvers.each do |user|
+      Dmail.create_automated(to: user, title: "You will lose approval privileges soon", body: <<~BODY)
+        You've approved fewer than #{MINIMUM_APPROVALS} posts in the past
+        #{APPROVAL_PERIOD.inspect}. You will lose your approval privileges in
+        #{days_until_next_month} #{"day".pluralize(days_until_next_month)}
+        unless you have approved at least #{MINIMUM_APPROVALS} posts by the end
+        of the month.
+      BODY
+    end
+  end
 end

app/logical/danbooru_maintenance.rb

@@ -21,6 +21,7 @@ module DanbooruMaintenance
   def weekly
     safely { UserPasswordResetNonce.prune! }
     safely { TagRelationshipRetirementService.find_and_retire! }
+    safely { ApproverPruner.dmail_inactive_approvers! }
   end
 
   def monthly

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