Make ActiveStorage/ActionText detection less likely to cause false positives

Closes #3659

Author: mshibuya

app/assets/javascripts/rails_admin/application.js.erb

@@ -21,10 +21,10 @@
 //=  require 'rails_admin/ui'
 //=  require 'rails_admin/custom/ui'
 
-<% if defined?(ActiveStorage) %>
+<% if defined?(ActiveStorage::Engine) %>
 //= require activestorage
 <% end %>
-<% if defined?(ActionText) && Rails.gem_version >= Gem::Version.new('7.0') %>
+<% if defined?(ActionText::Engine) && Rails.gem_version >= Gem::Version.new('7.0') %>
 //= require trix
 //= require actiontext
 <% end %>

app/assets/stylesheets/rails_admin/application.scss.erb

@@ -30,6 +30,6 @@
 @import "rails_admin/styles/base/theming";
 @import "rails_admin/custom/theming";
 
-<% if defined?(ActionText) && Rails.gem_version >= Gem::Version.new('7.0') %>
+<% if defined?(ActionText::Engine) && Rails.gem_version >= Gem::Version.new('7.0') %>
 @import "trix";
 <% end %>