Optimize organization_user creation on member creation

What does this MR do and why?

Optimize organization_user creation on member creation

• switch to use a find first and create if not found paradigm so that we don't waste id allocation
• follows https://docs.gitlab.com/ee/development/sql.html#find_or_create_by-is-not-atomic - not, see !142600 (comment 1743065026)
• see #439062 (closed)

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Related to #439062 (closed)