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.
In order to allow a user to notify his contacts of his migration
with a trustworthy signature the old private key must be available
to the new pod where we import data.
