Migrations: embed default thumbnail
Migrations: embed default thumbnail

The existing command expects to find the default thumbnail file in the
Archivematica shared directory, see: https://git.io/vHD7L. This migration
attempts to remove that level of coupling by introducing a new version of the
command as a Python script where the contents of default.jpg are embedded.

artefactual/archivematica-fpr-admin,artefactual/archivematica-fpr-admin,artefactual/archivematica-fpr-admin,artefactual/archivematica-fpr-admin