Move EE prepend/include to the end of mailers

This moves all instances of `prepend EE::Something` and `include
EE::Something` in mailers to the last line of the corresponding file.
This pushes EE specific code further down the files, reducing the
likelihood of developers running into merge conflicts.

app/mailers/previews/notify_preview.rb

 # frozen_string_literal: true
 
 class NotifyPreview < ActionMailer::Preview
-  prepend EE::Preview::NotifyPreview
-
   def note_merge_request_email_for_individual_note
     note_email(:note_merge_request_email) do
       note = <<-MD.strip_heredoc
@@ -196,3 +194,5 @@ class NotifyPreview < ActionMailer::Preview
     email
   end
 end
+
+NotifyPreview.prepend(EE::Preview::NotifyPreview)