Change delivery failure tracking to work with hostnames instead of URLs (#13437)

db/migrate/20200407202420_migrate_unavailable_inboxes.rb

@@ -0,0 +1,16 @@
+class MigrateUnavailableInboxes < ActiveRecord::Migration[5.2]
+  disable_ddl_transaction!
+
+  def up
+    urls = Redis.current.smembers('unavailable_inboxes')
+
+    urls.each do |url|
+      host = Addressable::URI.parse(url).normalized_host
+      UnavailableDomain.create(domain: host)
+    end
+
+    Redis.current.del(*(['unavailable_inboxes'] + Redis.current.keys('exhausted_deliveries:*')))
+  end
+
+  def down; end
+end