Avoid using live models in old migrations

Instead, we define models in the migrations themselves. This ensures that
column names etc match the state of the database during the migration,
not the current live version of the model.

db/migrate/20180204153242_tile_users.rb

@@ -1,4 +1,7 @@
 class TileUsers < ActiveRecord::Migration[5.1]
+  class User < ActiveRecord::Base
+  end
+
   def up
     add_column :users, :home_tile, :bigint
     add_index :users, [:home_tile], :name => "users_home_idx"