Problem: can't link existing problems to each other

A "child" problem is a dependency of a parent problem. This is an explicit and direct relationship. A parent cannot be closed unless all children are closed.

But sometimes you go to log a problem, and find out that someone else has already logged it under a different parent. In this situation, the child problem is a dependency of both parents.

To avoid needing to log the same problem multiple times, we should be able to simply link any problem as a dependency of any other problem.

Caveat: need to prevent circular dependencies.