Move Fluent files from "browser" to "toolkit"

I realised in #41622 (closed) that I included "browser" Fluent paths in the "toolkit" neterror pages.

As far as I remember, "about:neterror" is not used on Android, but it might be a good idea to follow the convention and move the Fluent files ("tor-browser.ftl" and "base-browser.ftl") to the "toolkit" directory.

@pierov what do you think?