Extract email-ownership check to `Group#owner_of_email?`

What does this MR do and why?

In #385785 (closed) we need to implement a condition to check whether a user should be an Enterprise User, see https://about.gitlab.com/handbook/support/workflows/gitlab-com_overview.html#enterprise-users. As a part of the check, we will need to check email ownership. This refactoring would allow reusing existing implementation, see !95407 (merged).

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