fix(repo): unify DEFAULT_SHOW_FULL_NAME output in templates and dropdown (#36597)

The design of DefaultShowFullName has some problems, which make the UI
inconsistent, see the new comment in code

This PR does a clean up for various legacy problems, and clarify some
"user name display" behaviors.

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Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>

templates/repo/issue/filter_item_user_assign.tmpl

@@ -10,7 +10,7 @@
 {{$queryLink := .QueryLink}}
 <div class="item ui dropdown jump {{if not .UserSearchList}}disabled{{end}}">
 	{{$.TextFilterTitle}} {{svg "octicon-triangle-down" 14 "dropdown icon"}}
-	<div class="menu">
+	<div class="menu flex-items-menu">
 		<div class="ui icon search input">
 			<i class="icon">{{svg "octicon-search" 16}}</i>
 			<input type="text" placeholder="{{ctx.Locale.Tr "repo.issues.filter_user_placeholder"}}">