[web][landing] Add legal acknowledgements to SIWE flow

Summary:
Context: https://linear.app/comm/issue/ENG-2835/disable-auto-accepting-policies-after-siwe
Add Terms of Use and Privacy Policy acknowledment to SIWE flow.

Test Plan:
Looks as expected:

Reviewers: ashoat, tomek, kamil

Reviewed By: ashoat

Differential Revision: https://phab.comm.dev/D6406