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.

apps/files/static/files/upload.js

@@ -253,6 +253,16 @@ class MultipleFilesWidget {
 }
 
 $(function() {
+    // Display OS-specific stuff
+    function os_class() {
+        if (navigator.platform.indexOf("Win") != -1) return "os-windows";
+        if (navigator.platform.indexOf("Mac") != -1) return "os-mac";
+        return "os-other";
+    }
+
+    document.querySelector('body').classList.add(os_class())
+
+    // Get the CSRF token for form submitting
     let csrf_token = document.querySelector("[name=csrfmiddlewaretoken]").value
 
     window.images = new MultipleFilesWidget(