Nicely handle missing user in collaborations (#17049) (#17166)

Backport #17049

It is possible to have a collaboration in a repository which refers to a no-longer
existing user. This causes the repository transfer to fail with an unusual error.

This PR makes `repo.getCollaborators()` nicely handle the missing user by ghosting
the collaboration but also adds consistency check. It also adds an
Access consistency check.

Fix #17044

Signed-off-by: Andrew Thornton <art27@cantab.net>

Co-authored-by: KN4CK3R <admin@oldschoolhack.me>

models/access.go

@@ -225,6 +225,9 @@ func (repo *Repository) refreshCollaboratorAccesses(e Engine, accessMap map[int6
 		return fmt.Errorf("getCollaborations: %v", err)
 	}
 	for _, c := range collaborators {
+		if c.User.IsGhost() {
+			continue
+		}
 		updateUserAccess(accessMap, c.User, c.Collaboration.Mode)
 	}
 	return nil