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
- Blocks tails/accounting#15507