Add link to a resource's nested stack

When viewing a resource, it's valuable to know whether the resource is a
nested stack with additional resources attached to it.  This change
provides a new 'nested' link in the links section of a resource, which
points to the nested stack.

Co-Authored-By: Richard Lee <rblee88@gmail.com>
Implements: blueprint nested-stack-link
Change-Id: Ibc8768d2be8f1dae9a60d3818ae9f56c27bbdc9b
Merge "Add link to a resource's nested stack"