Implement client migration for rebranding and multiple pool networks support upgrade

When CLI is being started for the first time (per user) after the rebranding upgrade the migration mechanism must check whether the current user's home directory contains .sovrin subdirectory. If the user's home directory contain .sovrin subdirectory, then the migration mechanism must offer the user to migrate this data. If the user agrees then the application data must be migrated from .sovrin subdirectory to .indy subdirectory and the former must be kept as a backup.

See comments to INDY-830 for details on migration for client.

UPD:
In scope of this task a client migration for the indy-node version that supports multiple pool networks (see INDY-832, INDY-833 for details) must also be implemented. This client migration may be combined with the client migration for the rebranded indy-node version (because both rebranding and multiple pool networks support will be introduced simultaneously in the upcoming stable indy-node version).