expand @username values for author and committer into emails

Created by: camdencheek

This adds support for @username syntax for author: and committer: to the new commit/diff search path, expanding sourcegraph usernames into emails to search git with.

Deciding where this code should live was a little weird, since we'd like to construct commit search jobs as statically as possible. I settled on a "construct the static job first, then use a runtime method to expand the values" approach. This keeps the NewSearchJob signature from exploding, and avoids adding any runtime constructs like ctx and db to the function. I'd be interested in other ideas if someone has a better way to handle this though.

Fixes #25180 (closed)