Disable Autocrypt by default

Originally created by @hefee on #16186 (Redmine)

Currently Autocrypt breaks workflows.

There are currently two settings, we should update:

Disable Autocrypt for Tails users by default.
And if users enable Autocrypt “Prefer encrypted emails from the people you exchange email with.” should be enabled by default in Tails. Autocrypt communicate with this setting, that the Recipient should use prefer encryption.

Feature Branch: bugfix/16186-thunderbird-disable-autocrypt

Edited May 15, 2020 by hefee

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