[lib/native/web] introduce pinnedMessageCountText function

Summary:
Noticed that this logic was being repeated in several places + was about to reintroduce this logic again in another place. Rather than copy+paste figured it would be better to factor this logic out into it's own function

Depends on D10346

Test Plan: flow and confirmed both on native and web that there are no regressions with the pinned message count text

Reviewers: atul, rohan, kamil

Reviewed By: rohan

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D10347