Add IST balance and 'Get IST' button to smart wallet provision dialog

[otoole-brendan]

#116 adds the auto-wallet provisioning flow to the PSM.

This ticket improves the UX but adding two features to the dialog:

1. The user's IST balance
2. A 'Get IST' button that launches Leap Elements modal for easy IST onboarding (to provision smart wallet)

As part of this dialog change - 'Proceed' should not be illuminated or clickable until the user has at least sufficient IST balance to cover wallet-provisioning (1IST).

The Leap Elements modal should include the Kado module to ensure users with no crypto assets have a means to buy IST using fiat/credit card

Design

Match to dapp-inter dialog per Agoric/dapp-inter#227 (comment)

[samsiegart]

completed: #128