Install Enigmail from Buster

With #16738 (closed), we pinned Enigmail to Bullseye, to have a version that is not vulnerable to https://security-tracker.debian.org/tracker/CVE-2019-12269.

We should revert commit e812b16f9433db21401ae59f7fa352bd16145144 once this issue is fixed in Enigmail in Buster.

Feature Branch: bugfix/16978-install-enigmail-from-buster

Original created by @segfault on 16978 (Redmine)

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