Show full name if DefaultShowFullName setting activated (#6710)

Adds a new key DEFAULT_SHOW_FULL_NAME (default false) to the [ui] section.
If enabled the full name will be shown (unless it's empty, then
the default username will be used)

templates/repo/diff/comments.tmpl

@@ -7,7 +7,7 @@
 	</a>
 	<div class="content">
 		<div class="ui top attached header">
-			<span class="text grey"><a {{if gt .Poster.ID 0}}href="{{.Poster.HomeLink}}"{{end}}>{{.Poster.Name}}</a> {{$.root.i18n.Tr "repo.issues.commented_at" .HashTag $createdStr | Safe}}</span>
+			<span class="text grey"><a {{if gt .Poster.ID 0}}href="{{.Poster.HomeLink}}"{{end}}>{{.Poster.GetDisplayName}}</a> {{$.root.i18n.Tr "repo.issues.commented_at" .HashTag $createdStr | Safe}}</span>
 			<div class="ui right actions">
 			{{if and .Review}}
 				{{if eq .Review.Type 0}}