April 2021: The 'Respecting Boundaries' release

Main addition (and cause of some fairly major code shuffling) in this release is Packwerk, to enforce plugin boundaries.

Also added routes partials, bullet (N+1 query detection), CodeClimate coverage reporting, and the Items extension for Pagy.

Plus lots of improvements to base controllers and mailers (now in the core plugin's app/public area), and moved Rails Email Preview engine, rubocop config, and ShinyCMS rake tasks, from main_app into the core plugin.