"Scroll" gutter by relatively positioning its line numbers

This matches how we scroll lines, and eliminates opportunities for the gutter to get out of sync with the lines. If that happened, it would probably be a bug, but this at least eliminates one source of confusion when debugging.

static/editor.css

@@ -27,6 +27,10 @@
   text-align: right;
 }
 
+.editor .gutter .line-numbers {
+  position: relative;
+}
+
 .editor.mini .gutter {
   display: none;
 }