Translation copyright

[Julien Lepiller]

Hi Guix!

I recently submitted a request to savannah admins to open a new project
for our translations (as we discussed back in February, so weblate can
have access to it, but not to the main repos). Amin was kind enough to
answer and noticed something weird with our copyright headers. We have
a lot of different headers in there.

Some of them are "Copyright (C) 2020 Ludovic Courtès" when Ludo
probably never modified the file himself (eg: guix-cookbook.de.po),

Others are "Copyright (C) 2018-2020 the authors of Guix (msgids)" (eg:
guix-manual.de.po)

Some are "Copyright © 2013 Free Software Foundation, Inc." (eg:
packages/vi.po)

Actual authors of translations are listed in the file, because the TP
robot would not accept the file otherwise.  However, the first copyright
header is more related to msgids and should be the same everywhere
(except for years).

I don't think Ludo is the only copyright holder here, and since we
don't require contributors to assign copyright to the FSF, I don't
think the copyright goes to the FSF either.

I'd like to change all of these headers to something like this:

Copyright (C) 2018-2020 the authors of Guix (msgids)

Followed by the copyright of individual translators (or maybe the
previous line is enough?)

This is probably the header to use in the pot files too.

Thank you!