Prepend AppSubUrl to links for default avatar (#15341) (#15342)

Backport #15341

Fix #15334

Signed-off-by: Andrew Thornton <art27@cantab.net>

templates/repo/diff/comments.tmpl

@@ -3,7 +3,7 @@
 {{ $createdStr:= TimeSinceUnix .CreatedUnix $.root.Lang }}
 <div class="comment" id="{{.HashTag}}">
 	{{if .OriginalAuthor }}
-		<span class="avatar"><img src="/img/avatar_default.png"></span>
+		<span class="avatar"><img src="{{AppSubUrl}}/img/avatar_default.png"></span>
 	{{else}}
 		<a class="avatar" {{if gt .Poster.ID 0}}href="{{.Poster.HomeLink}}"{{end}}>
 			{{avatar .Poster}}