Investigate release timing of Icedove

Originally created by @Anonymous on #10753 (Redmine)

1. What’s the plan regarding having MFSA’s fixed in Icedove when we release a new Tails? In other words: will we always be lagging behind security updates in Icedove by N weeks, or can we do better?

1. Does upstream release a new Thunderbird at the same time as the new Firefox?
   See:
   https://www.mozilla.org/en-US/thunderbird/releases/
   https://www.mozilla.org/en-US/firefox/releases/
   https://wiki.mozilla.org/Releases

1. How fast does it flow into Debian?
   See https://tracker.debian.org/pkg/icedove

Parent Task: #5663 (closed)