Epic migration should include system notes

Problem to Solve

With the implementation of the Epic migration MVC we need to migrate over system notes of the epic.

Proposed Solution

Update lib/gitlab/import_export/group/import_export.yml to include system_note_metadata, which should start including system notes in the export/import of group epics.

Documentation

• User docs: https://docs.gitlab.com/ee/user/group/import/ (WIP)
• Dev docs: https://docs.gitlab.com/ee/development/bulk_import.html