MythArchive: fix the import/export of recordings to the native archive format

* This updates the native import/export to work with the current DB table
  fields. Fixes #10871

* Add storage group support so the FE running MythArchive no longer needs
  local access to the recordings. Fixes #8657.
(cherry picked from commit 003f3023f2483f88f84df73b41fe2f976bb119d0)