[landing] Rename SIWENonceContext to SIWEContext

Summary:
This React.Context was initially just to make the siweNonce included in the request header available in the siwe.react.js component without a bunch of prop drilling.

However, going forward we're also going to need to make siwePrimaryIdentityPublicKey accessible to the siwe.react.js component. Instead of introducing yet another React.Context it makes sense to turn SIWENonceContext into a more general SIWEContext that can handle both the nonce and public key.

This diff is solely about renaming the context in place to reduce clutter in subsequent diffs.

Test Plan: SIWE flow continue to work as expected.

Reviewers: ashoat, tomek

Reviewed By: ashoat

Differential Revision: https://phab.comm.dev/D6127