[web] Flip the switch to enable SIWE on web

Summary:
Remove the isDev checks from LoginForm component and display either

A. TraditionalLoginForm and SIWEButton or
B. SIWELoginForm and "back button"

Depends on D6639

Test Plan: Things continue to work as expected.

Reviewers: ashoat, tomek

Reviewed By: ashoat

Differential Revision: https://phab.comm.dev/D6640