Fix some old migration scripts (#17394)

* Fix some old migration scripts

* Fix edge case in two-step migration from older releases

db/migrate/20181026034033_remove_faux_remote_account_duplicates.rb

@@ -1,6 +1,46 @@
 class RemoveFauxRemoteAccountDuplicates < ActiveRecord::Migration[5.2]
   disable_ddl_transaction!
 
+  class StreamEntry < ApplicationRecord
+    # Dummy class, to make migration possible across version changes
+    belongs_to :account, inverse_of: :stream_entries
+  end
+
+  class Status < ApplicationRecord
+    # Dummy class, to make migration possible across version changes
+    belongs_to :account, inverse_of: :statuses
+    has_many :favourites, inverse_of: :status, dependent: :destroy
+    has_many :mentions, dependent: :destroy, inverse_of: :status
+  end
+
+  class Favourite < ApplicationRecord
+    # Dummy class, to make migration possible across version changes
+    belongs_to :account, inverse_of: :favourites
+    belongs_to :status,  inverse_of: :favourites
+  end
+
+  class Mention < ApplicationRecord
+    # Dummy class, to make migration possible across version changes
+    belongs_to :account, inverse_of: :mentions
+    belongs_to :status
+  end
+
+  class Notification < ApplicationRecord
+    # Dummy class, to make migration possible across version changes
+    belongs_to :account, optional: true
+    belongs_to :from_account, class_name: 'Account', optional: true
+    belongs_to :activity, polymorphic: true, optional: true
+  end
+
+  class Account < ApplicationRecord
+    # Dummy class, to make migration possible across version changes
+    has_many :stream_entries, inverse_of: :account, dependent: :destroy
+    has_many :statuses, inverse_of: :account, dependent: :destroy
+    has_many :favourites, inverse_of: :account, dependent: :destroy
+    has_many :mentions, inverse_of: :account, dependent: :destroy
+    has_many :notifications, inverse_of: :account, dependent: :destroy
+  end
+
   def up
     local_domain = Rails.configuration.x.local_domain
 

db/migrate/20190715164535_add_instance_actor.rb

@@ -1,4 +1,9 @@
 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 }
+  end
+
   def up
     Account.create!(id: -99, actor_type: 'Application', locked: true, username: Rails.configuration.x.local_domain)
   end

db/migrate/20191007013357_update_pt_locales.rb

@@ -1,4 +1,8 @@
 class UpdatePtLocales < ActiveRecord::Migration[5.2]
+  class User < ApplicationRecord
+    # Dummy class, to make migration possible across version changes
+  end
+
   disable_ddl_transaction!
 
   def up