Feature: Schema migrations for apps

[ShishKabab]

As apps evolve, they will change their schema. They could migrate their data themselves, but the problem is that other apps might rely on older versions of their schema. Therefore, using the storex-schema-migration package, we should allow users to describe their migrations in a declarative way. This means that 1) Storex Hub can execute them however it wants, and 2) it may be possible to automatically translate queries on older schemas to queries for the new schema (to be investigated). Another option would be delegate the responsibility for this backward compatibility to the apps themselves, but this would be sub-optimal for various reasons.