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