Move enums to model classes

Signed-off-by: Jiří Janoušek <janousek.jiri@gmail.com>
Rename Category model to Group

Signed-off-by: Jiří Janoušek <janousek.jiri@gmail.com>
Add BlockchainWalletImporter

Signed-off-by: Jiří Janoušek <janousek.jiri@gmail.com>
Ensure models are imported during schema creation

Signed-off-by: Jiří Janoušek <janousek.jiri@gmail.com>
Add ImportContactsCommand (CLI)

Signed-off-by: Jiří Janoušek <janousek.jiri@gmail.com>