[web] Parse invite link secret

Summary:
After user clicks invite link, we're checking if the url contains a secret and save it in redux if that's the case.

Depends on D7948

Test Plan: Open localhost:3000/invite/abc123 and check if navInfo in redux contains inviteSecret: "abc123".

Reviewers: bartek, kamil, inka

Reviewed By: inka

Subscribers: ashoat, atul

Differential Revision: https://phab.comm.dev/D7949