[lib] Make checkOnlyViewerBlock argument of threadIsWithBlockedUserOnly required

Summary:
I want to add another skipMemberAdminRoleCheck argument to this function. Can't have two optional arguments, so going to make them both required.

Only two callsites to threadIsWithBlockedUserOnly and they're pretty straightforward.

Depends on D12149

Test Plan: Trusting flow here.

Reviewers: ashoat, ginsu, tomek, varun

Reviewed By: ashoat

Differential Revision: https://phab.comm.dev/D12150