Fix autoloading in zeitwerk mode

[gburgett]

• Switches to use const_get which works in both classic and zeitwerk mode.
• Added doc about a pitfall of Zeitwerk autoloading with app-defined model namespaces

Rails 6 introduced the Zeitwerk autoloader which uses a completely different technique to load application models. The technique no longer depends on const_missing and in fact that method no longer works when a rails application is in :zeitwerk mode.

Instead, Zeitwerk depends on ruby's core "autoload" feature. It sets up the relationship between constants and files beforehand, instead of discovering them on-demand. The consequence of this is that const_get still works to get the constants, but Ruby (not Zeitwerk) auto-loads it on demand from the file. To the application, the constant exists regardless of whether it has been loaded or not.

More info: https://www.urbanautomaton.com/blog/2020/11/04/rails-autoloading-heaven/

[watermarkchurch-bot]

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[gburgett]

@gibsorya please release a new patch version of wcc-contentful once you've confirmed that it works in paper-signs