Upgrade to Thunderbird 60 final

In Tails 3.9~rc1 we’re going to ship 60.0~b10 but hopefully 60.0 or 60.1 will be packaged for Debian in time for Tails 3.9 final.

Feature Branch: feature/15792-thunderbird-60.0+force-all-tests, icedove:feature/15792-thunderbird-60.0

Original created by @intrigeri on 15792 (Redmine)

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