[Flow202][landing][skip-ci] [2/2] Fix Flow issues in SIWE code
Summary:
Separating out the SIWE code since I think different reviewers make sense.
NOTE: CI will fail on this diff. I considered the possibility of fixing Flow errors BEFORE upgrading Flow, but it wasn't possible... in some cases, the fixes to support the new version of Flow caused errors in the old version. I could have hidden these type errors with $FlowFixMe lines and then later revert those, but that seemed like too much busy work.
Depends on D9719
Test Plan: Make sure all Flow errors in landing are resolved
Reviewers: atul
Reviewed By: atul
Subscribers: tomek, wyilio
Differential Revision: https://phab.comm.dev/D9720