Support a link that, when clicked, will start a gitpod instance but with a PR's branch checked out instead of master. Will allow others to test a PR easily.

You can setup environment variables on Gitpod in the url like this:

- https://gitpod.io/#https://github.com/lichess-org/lila-docker
+ https://gitpod.io/#lila-pr=12345/https://github.com/lichess-org/lila-docker

Then, if lila-pr is set, we can fetch and checkout during setup.

git fetch upstream pull/12345/head:preview
git checkout preview