Move include to prepended block to fix service desk emails

What does this MR do?

Move include to prepended block to fix emails

Due to some magical interaction between ActiveSupport::Concern and ActionMailer::Base, we'll need to move them into the prepended block, otherwise those email methods cannot be properly promoted as class methods for Notify.

What are the relevant issue numbers?

Closes #7846 (closed)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Tests added for this feature/bug

Do we need a changelog entry? This is only on RC1