Assign ancestor group labels and filter by ancestor/descendent group labels

In the object (issue or merge request)

• Given I am in an issue or merge request, assign any group label from:
  • immediate parent group
  • all ancestors groups
• This includes sidebar assignment in an issue or merge request.
• This includes sidebar assignment in issue board card.
• Quick actions should be supported.

In group or project, issue or merge request lists

• Filter by any group label from:
  • immediate parent group
  • all ancestors groups
  • current group
• Additionally, in a group's issue or merge request lists, filter by any group label from any descendent groups
  • This case makes sense because now that we have #30106 (closed) and #39388 (closed), a group list can see descendent objects. So I should be able to filter by descendent group labels.

In project or group issue boards

• In the filter/search bar, and in the board config, filter by any group label from:
  • immediate parent group
  • all ancestors groups
  • current group
• Additionally, in a group's issue board filter/search bar, and in the board config, filter by any group label from any descendent groups This requirement is not part of this issue anymore, and will be worked on in https://gitlab.com/gitlab-org/gitlab-ce/issues/44723. This is as per https://gitlab.com/gitlab-org/gitlab-ce/issues/40915#note_65414126
• Add any group label list/column from:
  • immediate parent group
  • all ancestors groups
  • current group
• Additionally, in a group's issue board, and in the board config, add any group label list/column from any descendent groups Not correct per https://gitlab.com/gitlab-org/gitlab-ce/issues/40915#note_65034795.
• In a group's issue board, you can only add lists/columns that correspond to labels from your own group, or from your ancestor groups.

API

• Equivalent API updates so that when I object lists, I can apply those labels.