Demo apps (examples/example-web-app, examples/example-react-native-app) should disable buttons when a session is active

[sdlaver]

When a session is active from either of these two demos to a wallet, the user can switch back to the dapp and attempt to establish another session. This will fail, since a connection has already been established. We should disable the buttons in the demo app UI to indicate that you cannot establish another connection while the first is still in progress.