Add username choice for sign buffer requests

[eonwarped]

This will allow for websites to allow users to select the user from a dropdown instead of specifying the user ahead of time.

The response's data field will have the chosen username.

Example page is hosted https://eonwarped.github.io/steem/sk/main.html as well.

[eonwarped]

Now that I did this, I realize that I would prefer to do something more generic:

Just something like "requestUser"

1. User selects user from dropdown
2. Username gets returned with available auths.

[eonwarped]

For now, just want to simplify the scope to just sign buffer. It will then be used as the source for getting an account and then requesting any extra steps afterwards.

Changed behavior so that there is no 'enforce username' toggle. Instead, the API is such that it will only 'enforce' if the username is provided. Otherwise it will show a dropdown.