[lib] Pull baseThreadIsWithBlockedUserOnly out of baseThreadIsWithBlockedUserOnly[WithoutAdminRoleCheck]

Summary:
Pull common logic out of the two functions to reduce duplication and make things more readable.

Depends on D12294

Test Plan: flow + close reading, should be super simple refactor.

Reviewers: ashoat, ginsu, tomek, varun

Reviewed By: ashoat, tomek

Differential Revision: https://phab.comm.dev/D12295