Enable sub-group nesting/hierarchy within Editorial Groups

[pgoussy]

Is your feature request related to a problem? Please describe. Follow-up from #2303 - Would it also be possible to enable nesting/hierarchy for editorial groups? As a use case example, see this manually-coded editor page, wherein Guest Editors and Former editors are subsections within Contributing Editors, and multiple Special Issue subsections will eventually appear within Guest Editors. Nesting of Editorial Groups would enable us to use Janeway's Editorial Team page to replicate the structure of this page AND allow us to display Janeway's profile-based info cards for each editor within each of those subsections, rather than having to code the whole thing in the HTML/rich text editor.

Describe the solution you'd like Perhaps a similar setup to the Top Level Nav Item drop-down in the Content Manager, which could allow us to select a "Parent" editorial group for each group that should be nested within another.