Implement moving account

As title. Use POST /api/pleroma/move_account with password and target_account fields to make the move.

• Already done Something to consider: the PATCH /api/v1/accounts/update_credentials endpoint ( !2773 (closed) ) takes ap ids instead of nicknames. Mastodon always uses nicknames for both adding aliases and moving accounts. Maybe it's beneficial to add another endpoint like /api/pleroma/add_alias that does take nicknames, in order to make user interaction more unified.