Upgrade Linux to 4.19.20+ for Tails 3.13

Originally created by @intrigeri on #16458 (Redmine)

Feature Branch: bugfix/16462-16458-stretch-9.8-linux-4.19.20+force-all-tests

Edited May 15, 2020 by intrigeri

To upload designs, you'll need to enable LFS and have an admin enable hashed storage. More information