Upgrade Thunderbird to 60.4

Originally created by @intrigeri on #16261 (Redmine)

Thunderbird 60.4.0 had no MFSA but according to some internal mailing list, the corresponding Firefox MFSA applies. https://www.mozilla.org/en-US/security/advisories/mfsa2018-30/#CVE-2018-12405 is probably the only really concerning one since we disable HTML by default.

Thunderbird 60.5.0 is being prepared.

Feature Branch: feature/16261-thunderbird-60.4+force-all-tests, icedove.git:tails/stretch

Edited May 15, 2020 by intrigeri

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