Fix old migration script depending on the StreamEntry model (#11278)

db/migrate/20180528141303_fix_accounts_unique_index.rb

@@ -12,6 +12,11 @@ class FixAccountsUniqueIndex < ActiveRecord::Migration[5.2]
     end
   end
 
+  class StreamEntry < ApplicationRecord
+    # Dummy class, to make migration possible across version changes
+    belongs_to :account, inverse_of: :stream_entries
+  end
+
   disable_ddl_transaction!
 
   def up