Add migration script to soledad-server package

Definition of done

A migration script is added to soledad-server package. This migration script MUST define which version it is able to migrate from/to.

discussion

instructions for migrating are tracked by the related platform issue platform#8427 (closed)

(a page about migrations should be added to soledad docs too).

(from redmine: created on 2016-08-29)

• Relations:
  • relates #8421 (closed)