Resolve ""Add Members" drop-down doesn't separate Name/Username"

What does this MR do?

It fixes + aligns the styling of multi select user items. Spacing + Bold + @ sign changed. I thought about aligning it with the styling of single select but the avatar has s24 on purpose so it looks like a deliberate decision to have the one line format (which makes sense cause you see more results at once).

Screenshots (if relevant)

Before : chrome_2017-06-22_19-37-51

After :

Does this MR meet the acceptance criteria?

What are the relevant issue numbers?

Closes #33981 (moved)

Edited by Tim Zallmann

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