Move Tablet breakpoint from 768 -> 920

Summary: Bumping this up higher since two column views get extra tight fast below 900 px. This felt most correct to me, dialing it back from first attempt at 960. Mostly I don't want to ever accidentally trigger it when I'm on the 12" MacBook. Ref T10926

Test Plan: Durable Column, Workboards, Dashboards, Tasks.

Reviewers: epriestley

Reviewed By: epriestley

Subscribers: avivey, Korvin

Maniphest Tasks: T10926

Differential Revision: https://secure.phabricator.com/D15960