[web] rename message results modal to be pinned messages modal

Summary:
Spoke to @rohan about this, but initially there was some intention to make the message results modal the same modal for message pinning and message search that never panned out. To make things less confusing for other devs decided to rename this to PinnedMessagesModal

Depends on D10344

Test Plan: flow

Reviewers: atul, rohan, kamil

Reviewed By: rohan

Subscribers: ashoat, tomek, rohan

Differential Revision: https://phab.comm.dev/D10345