[web] Conditionally display ConnectButton once nonce received from keyserver

Summary:
We'll request a nonce from the keyserver siwe_nonce endpoint once a wallet has been successfully connected.

We display an ActivityIndicator while we wait for the request to complete. If the request was successful, we'll display the ConnectButton. If not, the ActivityIndicator component will show an ! to indicate error. Error message + handling will be improved later in the stack.

Depends on D6221

Test Plan:
Here's what it looks like:

Reviewers: ashoat, tomek

Reviewed By: ashoat

Differential Revision: https://phab.comm.dev/D6225