This commit does three things:
1. Documents a method
3. Implies the question: "Do we want to use class_attribute in Forem's codebase?"
2. Switches from an inferrence to an explicit (and configurable)
In my experience, I want to favor "explicit" declarations instead of
inferring what they should be. In this case, the inferrence is perhaps
adequate. But as I look to `ApplicationController::PUBLIC_CONTROLLERS`,
I think that is a prime case for a `class_attribute`. (The `api_action`
happened to be the lowest hanging fruit to begin the conversation.)
We still need some clarity into the `verify_private_forem` method as it
looks like it's doing a few different things.
There is precedence for using `class_attribute` found in
[`UniqueCrossModelSlugValidator.model_and_attribute_name_for_uniqueness_test`][1] (also
introduced by me).
[1]:https://github.com/forem/forem/blob/main/app/validators/unique_cross_model_slug_validator.rb
faaf2ec028