Show all inherited labels in a subgroup and project

Problem To Solve

In deeply nested group/subgroup/project hierarchies, labels inherited from ancestors do not show up a deeply nested project's label list. This is confusing and it also prevents those projects from setting inherited labels to priority labels.

Solution

Show all inherited labels within a given subgroup or project label list.

The backend changes are behind the feature flag show_inherited_labels, which has been defaulted to on (!45161 (merged)).

Acceptance

• All inherited labels are visible in a subgroup or project (even those that are deeply nested)
• Some visual indication that denotes from where the label was inherited.
• Tests -- especially for the deeply nested projects and subgroups as this seems to be a common problem with other features that have similar inheritance behavior.