Enable/disable owner and repo projects independently (#28805)

Part of #23318 

Add menu in repo settings to allow for repo admin to decide not just if
projects are enabled or disabled per repo, but also which kind of
projects (repo-level/owner-level) are enabled. If repo projects
disabled, don't show the projects tab.


![grafik](b9b43fb4-824b-47f9-b8e2-12004313647c)

---------

Co-authored-by: delvh <dev.lh@web.de>

models/repo/repo.go

@@ -411,6 +411,11 @@ func (repo *Repository) MustGetUnit(ctx context.Context, tp unit.Type) *RepoUnit
 			Type:   tp,
 			Config: new(ActionsConfig),
 		}
+	} else if tp == unit.TypeProjects {
+		return &RepoUnit{
+			Type:   tp,
+			Config: new(ProjectsConfig),
+		}
 	}
 
 	return &RepoUnit{