(core) Add dark mode to user preferences

Summary:
Adds initial implementation of dark mode. Preferences for dark mode are
available on the account settings page. Dark mode is currently a beta feature
as there are still some small bugs to squash and a few remaining UI elements
to style.

Test Plan: Browser tests.

Reviewers: jarek

Reviewed By: jarek

Subscribers: paulfitz, jarek

Differential Revision: https://phab.getgrist.com/D3587

static/app.html

@@ -14,6 +14,7 @@
 <link rel="stylesheet" href="bootstrap-datepicker/dist/css/bootstrap-datepicker3.min.css">
 <link rel="stylesheet" href="bundle.css">
 <link rel="stylesheet" href="icons/icons.css">
+<!-- INSERT CONFIG -->
 <!-- INSERT CUSTOM -->
 
 <title><!-- INSERT TITLE --><!-- INSERT TITLE SUFFIX --></title>
@@ -57,8 +58,6 @@
     </tr></table>
   </div>
 
-  <!-- INSERT CONFIG -->
-
   <script src="jquery/dist/jquery.min.js" crossorigin="anonymous"></script>
   <script src="jqueryui/jquery-ui.min.js" crossorigin="anonymous"></script>
   <script src="bootstrap/dist/js/bootstrap.min.js" crossorigin="anonymous"></script>