Allow assigning and filtering issuables by ancestor group labels

closes #40915 (closed)

TODO list

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

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

• (to be done in https://gitlab.com/gitlab-org/gitlab-ce/issues/44723) Additionally, in a group's issue board filter/search bar, and in the board config, filter by any group label from any descendent groups

• 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 ancestor groups