Add manual "Connect Account" extension approval flow

[kwingram25]

The app should allow users to elect into approving access and adding accounts from the Polkadot-JS or Parity Signer extension, rather than on page load. In the design this can be done from the "Connect Account" button on the left sidebar, or a modal that is displayed on Rococo/other testnets when no user injected accounts are found. (previously part of #305)