Delete epic

• Delete epic.
• Only the group owner role can delete an epic.
• Show the button only if you have permissions.
• Have a dialog to confirm the delete.
• Have a confirmation horizontal banner to indicate the delete has occurred.
• Permissions: Only group owner can delete an epic.
• When an epic is deleted, then all the issues are no longe associated with that epic. This is an edge case that we should remember later on when we indicate within an issue who its parent epic is. For now, this should not matter, because when you assign one issue to another epic, it automatically moves over, and there is nothing blocking that from happening, per https://gitlab.com/gitlab-org/gitlab-ee/issues/3551. In the future this could be a consideration.

Designs

Once the epic is deleted, show a banner message similar to below, but with issue replaced with epic.