[landing] added argument to SIWE webview message

Summary: it was silly not to include this from the start - turns out it's easier to verify server-side when we pass the whole message along, not just the signature

Test Plan: n/a?

Reviewers: atul, ginsu, varun, ashoat

Reviewed By: ashoat

Subscribers: abosh, ashoat, tomek, varun, ginsu, atul

Differential Revision: https://phab.comm.dev/D5598