Re-enable Autocrypt

Originally created by @hefee on #16477 (Redmine)

As we had weird issues with Autocrypt:https://en.wikipedia.org/wiki/Autocrypt we decided within Tails 3.11 to disable Autocrypt for our users (#15923 (closed)). It lead our users to communicate unencrypted.

Let’s wait for some time and than recheck again, if Autocrypt is good enough for our needs.

So far, what we are missing:

If the user already encrypt communication with an other one, this should never fallback to unencrypted communication, when enabling Autocrypt.
“encrypt messages by default” should not been overwritten by Autocrypt.
Tails enable “Prefer encrypted emails from the people you exchange email with.” to hint Autocrypt to use Encryption.

Edited May 15, 2020 by hefee

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