Fix bookmarked (#486)

For the bookmarked class to be displayed in the right place, the parent should contain a relative position.

less/topic.less

@@ -101,6 +101,10 @@
 		border: 1px dashed @gray-light;
 	}
 
+	.post-header {
+		position: relative;
+	}
+	
 	.bookmarked {
 		position: absolute;
 		top: -2px;