[web] Pull SIWEButton out of SIWE component

Summary:
We introduce SIWEButton and continue to use as-is in SIWE component.

In subsequent diff we'll move SIWEButton out of SIWE into LoginForm so it can be used independently (just to initiate the SIWE auth flow).

Depends on D6488

Test Plan: This diff is a noop, tested the SIWE web flow and things continue to work as they did before.

Reviewers: ashoat, tomek, ginsu, rohan

Reviewed By: ashoat

Differential Revision: https://phab.comm.dev/D6489