[web] Pull SIWEButton out of SIWE component and use in LogInForm directly

Summary:
Pull SIWEButton out of SIWE component and use in LogInForm component directly. This allows us to render SIWEButton OR SIWE.

In following diffs will display traditional login form and SIWEButton OR just SIWE WITHOUT traditional login form.

Test Plan: If the user has a wallet connected (signer is truthy) and refreshes the page they will be shown SIWEButton and will need to click it again to see SIWELoginForm

Reviewers: ashoat, tomek

Reviewed By: ashoat

Differential Revision: https://phab.comm.dev/D6500