Epic: Require a Gitpod project for prebuilds

[jldec]

Now that projects are usable across git providers, Gitpod should limit prebuilds to repos with a project.

• [ ] Check for project before running prebuild
• [ ] Change documentation to explain that prebuilds require a project
• [ ] Post announcement about new prebuild requirement in changelog

[svenefftinge]

I checked the DB and there are a few projects that are active and don't have a project. We'd need to think about communication and test the upgrade path again. In order to take action now on the larger amount of prebuilds that are inactive I have created https://github.com/gitpod-io/gitpod/pull/9936

[stale[bot]]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.

[gtsiolis]

+1000 for this.