Improve right to left support in Thunderbird

Originally created by @goupille on #15764 (Redmine)

Apparently, using thunderbird in a language that requiers writing from right to left causes problems solved by the installation of this add-on :

since we’re preventing the installation of add-ons, a user suggested that we look into shipping this one in tails.

Edited May 15, 2020 by goupille

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