Bug 41428: Create Tor Browser's branding directories

Do not change Firefox branding, but create Tor Browser's branding directories, instead.

The first commit of the MR restores Firefox's branding directories, then the second adds our files to the directories we use (and cleans up the alpha branding). The third commit sets the branding of the dev builds to nightly (otherwise they're set to Firefox, but we could set to whatever you prefer, even release) and finally the fourth commit adds the new PDF icon from #41278 (closed).

Tested on Windows (both alpha and nightly testbuilds) and on macOS (nightly testbuild).

Closes #41428 (closed).

Needs tor-browser-build!619 (merged) (its tor-browser-build counterpart) to use the new directories.