Allow `@` using full name as well as username

[laryn]

Expected Behavior

When tagging another person using @ I would expect to be able to tag by name (whether full name or username). It would be nice to be able to just press "Enter" to select the first result.

Current Behavior

Right now it only searches the username, not the full name and the only way to select is by mouse click or arrowing down, I think.