Upgrade Thunderbird to 52.x

Tails 3.0 will ship Thunderbird 48.x. And icedove (1:52.1.1-1) just made it into the Debian NEW queue, woohoo! We should check if we have to upgrade to 52.x in Tails 3.1, or if this can wait until Tails 3.2, and draw plans accordingly. I’ll handle the initial assessment and will ask for help if needed.

Feature Branch: feature/12639-thunderbird-52

Related issues

Related to #13530 (closed)
Related to #12680 (closed)
Blocks #13234 (closed)

Original created by @intrigeri on 12639 (Redmine)

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