[landing] Reduce code duplication in landing/siwe.react.js

Summary: This code is completely duplicated from lib/utils/wagmi-utils.js.

Test Plan:

1. In my local environment, make sure that mobile SIWE still works with Rainbow wallet (mobile SIWE uses landing, but web SIWE doesn't)
2. In combination with other diffs in the stack, I tested the following on both my local environment, and in a staging environment for my keyserver on comm.domains:
   • Mobile sign in with Rainbow wallet using Rainbow login
   • Mobile sign in with Rainbow wallet using WalletConnect login
   • Web sign in with Rainbow wallet using Rainbow login
   • Web sign in with Rainbow wallet using WalletConnect login
   • Web sign in with Metamask wallet

Reviewers: rohan, atul, ginsu

Reviewed By: atul

Subscribers: tomek, wyilio

Differential Revision: https://phab.comm.dev/D9432