Move Rails engine hooks into after_initialize

[sco11morgan]

What

Move Rails engine hooks into after_initialize

Why

After adding action_markdown to a Rails 7.1 application that was already using sorcery the application would raise an error at startup.

active_support/inflector/methods.rb:290:in `const_get': uninitialized constant ActionMarkdown::TagHelper (NameError)

      Object.const_get(camel_cased_word)
            ^^^^^^^^^^

To be fair action_markdown works fine with stock Rails 7.1, and socery is known Sorcery/sorcery#308 (comment) to have Rails 7 compatibility problems. But hopefully this will at save someone a couple hours if they run into the same problem.