[web] Move setPrimaryIdentityPublicKey up from SIWELoginForm to LoginForm

Summary:
We're going to want the primaryIdentityPublicKey in both SIWELoginForm and TraditionalLoginForm, so we lift the useEffect and setting of primaryIdentityPublicKey "up" so it can be accessed by both child components.

Depends on D6675

Test Plan: primaryIdentityPublicKey continues to be included in SIWE message signing request.

Reviewers: ashoat, tomek

Reviewed By: ashoat

Differential Revision: https://phab.comm.dev/D6719