Button to provide new (unused) pubkeys from the brainkey

It would be nice to have a feature that allowed people to generate a set of fresh pubkeys from the brainkey.
This would allow them to easily replace account keys while being able to recover them from a brainkey.
It also enables to setup a new brainkey and migrate existing accounts to a new brainkey/wallet

The issue names no files, tests, or entry points. Start by locating the existing brainkey and account-key flows in bitshares-ui, then clarify how fresh pubkey generation, key replacement, and migration should work before defining completion criteria.

Written by the indexing model from the issue text.