Turns out that all we need is the path, and not the project id. The path
is way more user friendly, and we can easily get it from a CI variable
or combine it from the namespace & project name.
The GitLab project ID was still hardcoded to my playground project on
GitLab.com.
This commit instead reads from the predefined GitLab CI/CD variable for
the projects ID (`CI_PROJECT_ID`).
Makes navigating to the PR way easier when looking at the logs.
This requires a new `Forge.PullRequestURL()` method that needs to be
implemented for all forges.
