[web] Don't show TraditionalLoginForm once SIWE flow selected

Summary: We now have SIWE auth flow on a "separate page" that users can navigate to by hitting SIWEButton and can exit by hitting the "Go back" button.

Test Plan:
Works as expected (additional styling changes on the way):

Reviewers: ashoat, tomek

Reviewed By: ashoat

Differential Revision: https://phab.comm.dev/D6533