[web] Fix missing relationship buttons

Summary:
issue: ENG-7486
We might not have user infos in our UserStore, if we don't have any relationship with them. But we can extract them from the pending thread infos. useRelationshipPrompt takes pendingPersonalThreadUserInfo as an argument, and uses it if the user infos are indeed not present in the store.

Test Plan: Tested that the relationship buttons are present and work correctly

Reviewers: tomek, kamil

Reviewed By: tomek

Subscribers: ashoat

Differential Revision: https://phab.comm.dev/D11760