Adjust to Icedove → Thunderbird

https://lists.debian.org/debian-devel-announce/2017/02/msg00004.html has the details. We probably need to at least:

• drop the Claws Mail migration stuff that’s going to complicate things too much
• move and adjust any existing persistent ~/.icedove in live-persist, reusing code from the /usr/bin/thunderbird wrapper
• convert any existing persistent setting for ~/.icedove in live-persist
• adjust everything that’s about icedove in our config tree
• adjust our doc, including: rename pages, add redirects, mention the rename
• adjust our test suite
• mention the rename in the release notes

Beware, the packaging the config folder layout will change again

Feature Branch: tails:feature/11712-thunderbird t-p-s:master

Related issues

• Related to #12734 (closed)

Original created by @intrigeri on 12242 (Redmine)