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