closes #8336

We only store signatures for relayables if the author is external, but
if the author becomes external through a migration, the signature is
missing. Lets just use the old persons private key to still be able to
generate a signature for the export.

closes #8310

If the migration contains a new remote_photo_path migrate all photos of
the old person to this path. If the person was local before, cleanup old
uploaded files of the photos.

closes #8314

closes #8317

closes #8316

closes #8313

Update gems

See https://github.com/mperham/sidekiq/pull/4804

closes #8311

closes #8312

That way it can be re-used when sending the AccountMigrations to other
pods again if a message for the migrated account is received.

fixes #7902
closes #8309

closes #8307