Use latest form text

* Take the opportunity to reformat the source to not have so many hugely long lines.
* Put a few things through gettext that weren't going through it before
* Move 'Hold down Ctrl to select multiple files at once.' text to widget, out of
  help text for each individual control.

assets/css/ojuso.css

@@ -30,6 +30,11 @@ h1 {
 
 .mx-1 { margin-left: 10px; }
 
+.only-windows { display: none; }
+.os-windows .only-windows { display: initial; }
+
+.only-mac { display: none; }
+.os-mac .only-mac { display: initial; }
 
 /*
  * LAYOUT