[lib] Introduce shared getRelationshipActionText function

Summary: Moved part of the logic responsible for selecting correct text to relationship action button to lib, so it could be reused on web

Test Plan: Confirm mobile app relationship buttons in thread settings still contain correct text

Reviewers: tomek, atul

Reviewed By: tomek

Subscribers: ashoat, adrian, abosh

Differential Revision: https://phab.comm.dev/D4678