Select specific columns needed for Users in mailers [RUN ALL RSPEC] [RUN AS-IF-FOSS]

What does this MR do?

Related to #325332 (closed)

Seek and destroy User.find to cut down on select * from user and to select specific columns where feasible. I think mailers is a great candidate for this because:

• most of the time we just need :name
• we send tons of mail, especially issue and MR mailers

Screenshots (strongly suggested)

Does this MR meet the acceptance criteria?

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• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
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