Allow label references to specify a group

Currently, markdown references typically can only reference a group/project. You can't directly reference a group label - but can milestones references due to Allow milestone references to specify a full path (!143859 - merged). For example this would work, engineering/mobile/samsung/handy~"Samsung Label", but this engineering/mobile/samsubg~"Samsung Label" would not.

This leads to problem such as Milestones with duplicate name at different gro... (#424736 - closed)

header	header
- Engineering (group) - Mobile (group) - Mobile Milestone - iOS (project) - Milestone 1.2 - Android (project) - Milestone 1.2 - Samsung (group) - Samsung Label - Samsung Milestone - Handy (project) - Handy Milestone - Handy Label

Notice that for the group label ~Samsung Label, we can only reference it by referencing it in the project.