These used to say "Icedove (Thunderbird)" which wasn't handled so
nicely by s/Icedove/Thunderbird/.

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

Add sort="age" to inline directives in PO files, to avoid making hundreds of strings fuzzy (refs: #12053).