Investigation - Inheritance behaviour across Namespaces

Problem

There is a lot of inconsistency in our inheritance. See: #338819 (comment 667830288)

Example: How various plan objects currently behave

1. Labels from a parent Namespace are visible in a child Namespace label list.
2. Labels from a child are not visible in a parent's label list but they can be used for filtering & board lists.
3. Iterations from a parent Namespace are visible in a child Namespace.
4. Iterations from a child are not visible in the parent's iteration list.
5. Epics from a parent Namespace are visible in a child Namespace issue picker.
6. Epics from a parent are not visible a child Namespace epic list.
7. When an issue from a child is assigned to an epic in a parent, the epic is visible when "group by" is enabled.
8. Issues from a child can be assigned to milestones from a parent.
9. Issues from a child are visible in a parent's issue list.
10. Milestones from a parent are not visible in the child's Milestone list.
11. Milestones from a child are visible in the parent's Milestone list.
12. The Member of a child can view the child's work items within the parent Namespace Board and edit the work item from within this context even though they are not a Member of the parent Namespace.
13. There are all sorts of additional inconsistencies when it comes to Namespace sharing (&5791).

Proposal

1. Investigate the current state of inheritance of features and functionality across instance/group/subgroup/project.
2. Define and document general inheritance patterns for namespaces to follow going forward.