Add the ability to exclude remote users in user directory search results

[wrjlewis]

This change adds a new configuration user_directory.exclude_remote_users, which defaults to False.
When set to True, remote users will not appear in user directory search results.

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[wrjlewis]

This would fix #12958 and semi-relevant to #5301

[wrjlewis]

I think the failing tests are due to them being flakey. EDIT: passing.