Enabling custom deactivation email by default for SM

What does this MR do and why?

Describe in detail what your merge request does and why.

This MR enables the ability to customize deactivation emails by default for self-managed users. The Community contribution was originally being understood to target general custom emails but the final implementation is for deactivation emails only. It's behind the feature flag which can now be default enabled for users in 15.9 and removed in future milestone.

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