Allow signing-ui to work with tunnels

[shannonwells]

So that I can use the Signing UI from GitHub pages and test against a locally running node, I would like to be able to specify a tunnel URL such as from ngrok, e.g. wss://7768-104-13-63-142.ngrok-free.app in the "other endpoint" input box.

Possible solutions:

1. Add a checkbox that marks the other URL as a tunnel to one's locally running node. In such case we will redefine what "localhost" means in the code.
2. Add a checkbox that specifically says "Use Keyring for signing" so it will load the Alice - Ferdie accounts and use those for signing, instead of an extension,
3. Additionally to 2, maybe let you choose to use Keyring on any chosen endpoint.