DEV: Set disable_mailing_list_mode automatically (#12402)

The user mailing list mode continued to be silently enabled and
UserEmail job checked just that ignoring site setting
disable_mailing_list_mode.

An additional migrate was added to set disable_mailing_list_mode
to false if any users enabled the mailing list mode already.

db/migrate/20210315173137_set_disable_mailing_list_mode.rb

@@ -0,0 +1,21 @@
+# frozen_string_literal: true
+
+class SetDisableMailingListMode < ActiveRecord::Migration[6.0]
+  def up
+    result = execute "SELECT COUNT(*) FROM user_options WHERE mailing_list_mode"
+    if result.first['count'] > 0
+      execute <<~SQL
+        INSERT INTO site_settings(name, data_type, value, created_at, updated_at)
+        VALUES('disable_mailing_list_mode', 5, 'f', NOW(), NOW())
+        ON CONFLICT (name) DO UPDATE SET value = 'f'
+      SQL
+    end
+  end
+
+  def down
+    result = execute "SELECT COUNT(*) FROM user_options WHERE mailing_list_mode"
+    if result.first['count'] == 0
+      execute "DELETE FROM site_settings WHERE name = 'disable_mailing_list_mode'"
+    end
+  end
+end