Wagtailmenus 2.3.1 release notes

This is a maintenance release to fix to address the following:

  • Code example formatting fixes, and better use of headings in README.md.

  • Added ‘on_delete=models.CASCADE’ to all relationship fields on models where no ‘on_delete’ behaviour was previously set (Django 2.0 compatibility).

  • Marked a missing string for translation (@einsfr)

  • Updated translations for Lithuanian, Portuguese (Brazil), and Russian. Many thanks to @mamorim, @treavis and @einsfr!