[web] Replace IDENTITY_SERVICE_CONFIG with IDENTITY_SOCKET_ADDR

Summary:
Reasons to replace this:

1. determineSocketAddr is significantly simplified, without requiring much more complexity in the Webpack config code
2. We avoid the quirky behavior described here where webpack.DefinePlugin exposes JSON blob env vars as objects rather than strings, which doesn't match Flow's type for process.env
3. We avoid the distinction between null and undefined that spooked me here
4. This makes behavior more consistent with COMM_ALCHEMY_KEY and COMM_WALLETCONNECT_KEY here

Test Plan:
I tested with a keyserver/secrets/identity_service_config.json, then moved it to another path and tried again (to simulate when the value is not specified)

I used this patch to check the value: https://gist.github.com/Ashoat/f8ddc4061aeab0d45c477888c3a59678

Reviewers: varun, tomek

Reviewed By: varun

Differential Revision: https://phab.comm.dev/D10365