Mullvad Browser configuration files and branding

This MR adds a new mozconfig for Mullvad Browser and the various branding directories.

We still need the an SVG file with the "generic" logo, but apart from that, it should be ready.

9bc6a458 has been cherry-picked from 12.5, but we need it also for 12.0 (it was mainly done for MB).

dfa2b8e2 might be temporary, and it's actually been cherry-picked from tor-browser!521 (merged). We might want to wait for that to be actually merged before this one. Done

This MR is needed for #7 (closed), and solves #5 (closed), #12 (closed), #14 (closed), #18 (closed), #19 (closed), #35 (closed) and #38 (closed).

Partially solves #24 (closed) and #25 (closed) (it enables WebRTC at build time, but it doesn't enable it at runtime).