Fix instance actor being incorrectly created when running migrations (#18109)

* Add migration test about instance actor key

* Fix old migration

* Work around incorrect database state

db/migrate/20190715164535_add_instance_actor.rb

@@ -2,6 +2,14 @@ class AddInstanceActor < ActiveRecord::Migration[5.2]
   class Account < ApplicationRecord
     # Dummy class, to make migration possible across version changes
     validates :username, uniqueness: { scope: :domain, case_sensitive: false }
+
+    before_create :generate_keys
+
+    def generate_keys
+      keypair = OpenSSL::PKey::RSA.new(2048)
+      self.private_key = keypair.to_pem
+      self.public_key  = keypair.public_key.to_pem
+    end
   end
 
   def up