Add ability to turn off seat assignment duo emails for self-managed

What does this MR do and why?

Add ability to turn off seat assignment duo emails for self-managed

• customers want ability to turn off.
• default enable and give a switch to turn off as we want to give functionality here, not default turn it off.
• almost a reversion of !177462 (merged)
• conscious decision made to not use application setting.
• see https://gitlab.com/gitlab-org/gitlab/-/issues/557290

MR acceptance checklist

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How to replicate locally

same as these steps

Related to #557290