Use the https-everywhere Github repo

We're still using the repo on git.torproject.org, and it's obvious that https://git.torproject.org/https-everywhere.git is a remainder of earlier days. The new tag (2021.7.13) wasn't pushed to the git.tpo repo, so let's start pulling from Github.

To upload designs, you'll need to enable LFS and have an admin enable hashed storage. More information