[#143] Support categories tree structure (from Directory)
closes #143 (closed)
Base work: https://gitlab.cern.ch/push-notifications/web-portal/-/tree/146_rework_category_management_page Dependencies: https://gitlab.cern.ch/push-notifications/backend/-/merge_requests/149#7099facb0cf3af715c65ec403cc5c084f7103ca5
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Edited by Caetan Tojeiro Carpente