[web] Display thumbhash in chat

Summary: This adds the thumbhash to the chat messages while they're loading (both non-encrypted images and videos). More details and a demo video are provided in this Linear comment. Encrypted video support is added in D7900

Test Plan:
Throttled network and disabled cache in Chrome Dev Tools. Switched between chats with encrypted and non-encrypted media. The thumbhash was displayed for all images and videos while they were loading/decrypting.
See demo video in Linear comment.

Reviewers: ashoat, atul, ginsu, inka

Reviewed By: ashoat

Subscribers: tomek

Differential Revision: https://phab.comm.dev/D7902