[web] Open a community when a user clicks an invite link and is already a member
Summary:
Check if a user is already a member and navigate to a thread.
Test Plan:
Click a link to a community where a user is a member - check if a modal is displayed and after closing it a proper thread is displayed.
Click a link to a community where a user isn't a member - the old experience should be shown.
Reviewers: inka, ginsu
Reviewed By: inka
Subscribers: ashoat
Differential Revision: https://phab.comm.dev/D12316