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/search_name.tmpl

@@ -1 +1 @@
-<span class="gt-ellipsis">{{.Name}}{{if DefaultShowFullName}}<span class="search-fullname"> {{.FullName}}</span>{{end}}</span>
+<span class="username-display">{{.Name}} {{if .FullName}}<span class="username-fullname gt-ellipsis">({{.FullName}})</span>{{end}}</span>