[web] Have user sign SIWEMessage with wallet and hit siwe_auth endpoint

Summary: We prompt user to sign SIWEMessage with their wallet when "Sign in" is clicked. Once the message is signed, the user will be logged into the web app.

Test Plan: MetaMask:

Reviewers: ashoat, tomek

Reviewed By: ashoat

Differential Revision: https://phab.comm.dev/D6326