Disable Memory Hole for outgoing emails in Tails

As decided on #13649 (closed), we should disable memory hole for outgoing emails in Tails by default.
This is currently a non issue, because we use torbirdy/stable.

But we should be proactive and explicitly disable the pref now, so we don’t forget about it whenever we upgrade to Torbirdy 0.2.3 or newer for a totally unrelated reason (such as… switching to Debian testing :)

Feature Branch: feature/15201-disable-enigmail-memory-hole

Related issues

Related to #13649 (closed)
Blocks #13245 (closed)
Blocked by #15022 (closed)

Original created by @u on 15201 (Redmine)

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