Feature Request: enhance "sign-in" to support external wallets (e.g. Meta Mask / Trezor / Ledger).

Supporting external wallets should minimise the onboarding experience to (1) accept terms and conditions (2) type pin on external wallet (3) exchange.

External hardware wallets allow folks to maintain good levels of security even on potentially compromised computers, which increases confidence in using online systems.

The Kyber decentralised exchange is a great example:

The [Kyber web wallet source code](https://github.com/KyberNetwork/KyberWallet) is on github. Searching for, say, [Trezor](https://github.com/KyberNetwork/KyberWallet/search?utf8=%E2%9C%93&q=trezor&type=) shows that they are using the [Trezor Connect](https://github.com/trezor/connect) software.

Furthermore, [webauthn](https://www.w3.org/TR/webauthn/) is being developed to help standardise Trusted Platform Modules (TPMs) / Secure Elements (SEs) in web-browsers - where we might expect reduced development overhead in the future.

It would be great if BitShares had a similar experience to Kyber.

Further development of https://github.com/bitshares/bitshares-ui/issues/895.

Start by reading the earlier issue #895, then review the linked Kyber web wallet source and Trezor Connect documentation to understand the proposed wallet flow. Clarify which external wallets and browser capabilities BitShares should support; done should mean a defined, tested sign-in and exchange experience for the selected integrations.

Written by the indexing model from the issue text.