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 #9936

[stale]

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.