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

- Removed posts and non contacts from other's data
- Collections are exported in batches to lower memory footprint
- In base exporters create User object instead of keeping instance because it caches all associations

closes #7627

This commit introduces changes to the user data export archive format.
This extends data set which is included in the archive. This data can be
then imported to other pods when this feature is implemented.

Also the commit adds the archive format json schema. ATM it is used in
automatic tests only, but in future it will also be used to validate
incoming archives.