Bug 41370: Customize default bookmarks in tor-browser

We used to customize bookmarks in tor-browser-build.

For Windows and Linux we shipped our bookmark.html in the profile directory.

But macOS's profile directory works in a different way, so we ended up modifying language packs.

With this patch, we modify Firefox's default bookmarks file, and we don't need to do anything else in tor-browser-build.

As a result, we can use official and signed language packs on macOS, too.

Closes #41370 (closed).