[web] Use Redux instead of React.State for primaryIdentityPublicKey in SIWE component

Summary:
Use Redux instead of React.State for primaryIdentityPublicKey. Mostly so we can persist via redux-persist.

Depends on D6434

Test Plan: Monitored Redux DevTools and observed that actions fired as expected and that actions modified state as expected.

Reviewers: ashoat, tomek

Reviewed By: ashoat, tomek

Differential Revision: https://phab.comm.dev/D6451