This is just a massive 's/Icedove/Thunderbird/' both in terms of
content and filenames. No logic is changed, which should ease
reviewing of this massive commit.

Will-fix: #12242

This reverts commit 9e07c3d2.

Enabling the amnesia branding extension in Icedove for some reason makes
French the default spellchecker no matter the environment locale. As I
see no use of this extension in Icedove, let's just not enable it.

Will-fix: #11037

Will-fix: #9882