Check which version of Enigmail we should ship

Originally created by @intrigeri on #15657 (Redmine)

For #15602 (closed) we’re going to import Enigmail from sid into our custom APT repo (our freeze exception process). For 3.9 and later we should check whether we can install that package from stretch-security, or from sid, or update it in our custom APT repo, or something.

Feature Branch: hefee/bugfix/16186-disable-autocrypt+force-all-tests

Related issues

Related to #15602 (closed)
Related to #15923 (closed)
Related to #16120 (closed)
Blocks tails/accounting#15507

Edited May 15, 2020 by intrigeri

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