Consider different approach for "2176: Rebrand Firefox to TorBrowser "

The current patch replaces all occurrences of branding/brand.ftl with branding/tor-browser-brand.ftl. This means that many files are touched by the patch (increasing chances of rebase conflict), and whenever Firefox adds new instances of branding/brand.ftl we need to modify the patch to also cover those.

I think we should try a different approach to keep all instances of branding/brand.ftl untouched, and do the branding/brand.ftl -> branding/tor-browser-brand.ftl remapping somewhere else, and just in a single place.

One way would be to force the Fluent FileSource that we register in torbutton to take precedence over any other source and rename tor-browser-brand.ftl to brand.ftl, to override Firefox one (including langpacks).

We probably would need to do this in L10nRegistry.js, but I would need to investigate a bit more.