Replace TorBirdy's sanitizeHeaders() function
TorBirdy sanitizes the “Date” and “Content-Language” headers of outgoing emails, see https://gitweb.torproject.org/torbirdy.git/tree/components/torbirdy.js#n393.
We should assess whether we can replace that function somehow. I assume that this would require patching Thunderbird.
Parent Task: #17219
Original created by @segfault on 17221 (Redmine)