Only run load_subclasses after app is initialized

In rails 6.1, nothing was calling descendants or subclasses during the
app initialization.

Rails changed in 7.0 to call subclasses from reload_schema_from_cache here:
rails/rails@6f30cc0

It also changed to call descendants on the callback class (self) insead of
the ActiveSupport::DescendantsTracker here:
rails/rails@ffae3bd

We are not expecting to be called from these locations.

We can make this rails 7 compatible by ensuring the descendant loader loading
of subclasses until after the app is booted, which was the implicit behavior
previously.

lib/extensions/descendant_loader.rb

@@ -263,7 +263,7 @@ def scoped_name(name, scopes)
 
   module ArDescendantsWithLoader
     def descendants
-      unless defined? @loaded_descendants
+      if Vmdb::Application.instance.initialized? && !defined? @loaded_descendants
         @loaded_descendants = true
         DescendantLoader.instance.load_subclasses(self)
       end
@@ -275,7 +275,7 @@ def descendants
     # https://github.com/rails/rails/commit/8f8aa857e084b76b1120edaa9bb9ce03ba1e6a19
     # We need to get in front of it, like we do for descendants.
     def subclasses
-      unless defined? @loaded_descendants
+      if Vmdb::Application.instance.initialized? && !defined? @loaded_descendants
         @loaded_descendants = true
         DescendantLoader.instance.load_subclasses(self)
       end