Upgrade Linux to 4.12

Linux 4.11 is now in Debian testing. It’s likely it (or 4.12) will make it into stretch-backports in time for Tails 3.2. Let’s get ready for it.

Feature Branch: feature/12732-linux-4.12

Original created by @intrigeri on 12732 (Redmine)

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