[web] Refactor Mutlimedia to function component

Summary: Refactor the <Multimedia> component to be a function component. This will make managing further thumbhash logic much easier.

Test Plan: Multimedia are still displayed as before (both in chat and input bar). Clicking on the message opens modal. Remove button in input bar works. URL revoking is tested by console.log and ensuring it's called once when the message is sent.

Reviewers: ashoat, atul, ginsu, inka

Reviewed By: ashoat

Subscribers: tomek

Differential Revision: https://phab.comm.dev/D7899