Check which version of Enigmail we should ship

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 #15507 (closed)

Original created by @intrigeri on 15657 (Redmine)

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