[web] Pull TraditionalLoginForm out of LoginForm component

Summary: LoginForm was handling both "traditional" and SIWE login. In subsequent diffs we'll have a separate TraditionalLoginForm and SIWELoginForm component that are displayed conditionally.

Test Plan: Still a noop, both "traditional" and SIWE login continue to work as expected.

Reviewers: ashoat, tomek, ginsu, rohan

Reviewed By: ashoat

Differential Revision: https://phab.comm.dev/D6501