Add CLI command or extra script for converting keys

During signup, it's possible to create unencrypted JSON files for the role account. Those unencrypted files cannot be easily imported into the UI (see https://github.com/Joystream/apps/issues/108).

The CLI could easily provide commands or extra scripts for:

• Adding/removing passwords to/from JSON files.
• Extracting the raw seed from a JSON file.

Try to accept seeds for the member account, and try to display the actor account seeds, too.