Upgrade Thunderbird to 60.4

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

Original created by @intrigeri on 16261 (Redmine)

To upload designs, you'll need to enable LFS and have an admin enable hashed storage. More information