[web] Allow user to cancelSIWEAuthFlow from SIWELoginForm

Summary:
Allows user to "go back" to traditional login flow after they've initially selected the SIWE flow.

(Right now the traditional login flow is "sticky" and doesn't disappear once SIWE auth flow is selected, but it will disappear in subsequent diffs. For now the back button basically takes us back to SIWEButton in the siweSection of the LoginForm component.)

(Styling will also be cleaned up in subsequent diff)

Test Plan:
Able to select/deselect SIWE flow and things behave as expected:

Reviewers: ashoat, tomek

Reviewed By: ashoat

Differential Revision: https://phab.comm.dev/D6503