General strings cleanup before moving to Weblate.
I've also split some files, which might be a little problem when updating strings for 12.0 in the future.
Finally, I've tried to add copyright notices, I'm not sure I've done the right thing (e.g.,
brand.* are copied from Mozilla and updated, but it's the only way to do it, should have I left Moz's copyright?)
Closes #41375 (closed).