Upstream moved about:rights to a remote location in Bug 1946446

Bugzilla: Bug 1946446 - Update about:rights (commit ec4d54b6).

The outcome was that about:rights was removed, and about:rights now redirects to a mozilla.org page, like about:credits.

For now, I restored our about:rights page, but it doesn't work, as I think we were using some strings from upstream, which were also removed.

@ruihildt: how do you want us to proceed? To copy Moz's strings we used to our files, and keep the offline copy, or do you want to do the same as Mozilla, and send to an online version on your site?