Disable live previews on mobile

Summary:
Fixes T1895. Now that we have on-demand prviews, we can use them on mobile. On mobile:

  - don't show live previews;
  - only save drafts every 10 seconds.

Also, show fewer remarkup buttons on mobile to try to make sure the more important ones (preview, e.g.) fit.

Test Plan:
  - Made window narrower and wider to trigger preview/no-preview behavior.
  - Used DarkConsole to verify request rate.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T1895

Differential Revision: https://secure.phabricator.com/D14856

webroot/rsrc/css/core/remarkup.css

@@ -556,3 +556,7 @@ var.remarkup-assist-textarea {
 .remarkup-assist-button.preview-active:hover .phui-icon-view {
   color: {$lightsky};
 }
+
+.device .remarkup-assist-nodevice {
+  display: none;
+}