* `MysqlDateTimeWithTimeZone` inherits from `ActiveRecord::Type::DateTime`
  (`MysqlDateTime` is not present in Rails 5)
* explicitly set `NULL` default value for `merge_request_diff_files`'s
  `diff` column (otherwise empty string is used in a migration) and
  empty string is not allowed for text/blob fields in Mysql
* disable NO_ZERO_DATE mode for all Mysql DB connections, otherwise
  SQL queries fail on inserting `0` value for `created_at` column