Downcase invite emails as the service layer

What does this MR do and why?

Downcase invite emails as the service layer

• normalize the data for case sensitivity to match user and email model handling.
• see caveats to the current approach in #460708 (comment 1907853211) and #460708 (comment 1909418594)

see details in #460708 (closed)

MR acceptance checklist

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Related to #460708 (closed)