On certain migration instances (see the MigLeg and MigKnar test environments), the portals folder persists.

This folder was used in legacy, but isn't used in SGR.
Should probably be handled in some way (deleted)?

If some portals failed to migrate, the portal folder must persist, or their data must be stored in another file instead.

I think the name "Portals" is misleading for that purpose;
perhaps we can rename the folder "Archives" or something similar?

Or perhaps Legacy Data? We could put any invalid gate formats there as well.