Prioritize exact matching on username and e-mail for users search

What does this MR do and why?

!135917 (merged) fixed the users API so that exact matches would be returned first. However, the previous User#search would prioritize matches on the user's full name. However, on instances where there are many users of the same full name (e.g. Grzesiek), the sort order makes it hard to find the exact user. This change swaps the priority so that public e-mail, username, and full name are matched in that order. Since e-mails have a @ character, a match on an e-mail should always take priority.

Relates to #215499 (closed)

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