Ensure structure:load runs in single transaction

Add an initializer to set additional flag on db:structure:load that will
load the structure.sql file in a single transaction.

config/initializers/structure_load_in_transaction.rb

+# frozen_string_literal: true
+
+ActiveRecord::Tasks::DatabaseTasks.structure_load_flags ||= []
+
+flag = '--single-transaction'
+
+unless ActiveRecord::Tasks::DatabaseTasks.structure_load_flags.include?(flag)
+  ActiveRecord::Tasks::DatabaseTasks.structure_load_flags << flag
+end