[native, lib] Move thread members actions callbacks into shared lib function

Summary:
Move logic for two functions:

• removing user from thread
• switch user role between user and admin

into lib shares, so they could be used also in web app.

Test Plan: Promoting user to an admin and removing the user from a thread should work exactly as before

Reviewers: palys-swm, benschac, atul

Reviewed By: palys-swm

Subscribers: ashoat, palys-swm, Adrian, atul, karol-bisztyga, benschac

Differential Revision: https://phabricator.ashoat.com/D3381