[web] Add missing thumbhash to newly created media message

Summary:
This addresses https://phab.comm.dev/D7902#inline-52026.
I missed this change when implementing thumbhash generation on web.

Test Plan: Ensured with redux-devtools that the just created media message has thumbhash field set.

Reviewers: ashoat

Reviewed By: ashoat

Subscribers: tomek, atul

Differential Revision: https://phab.comm.dev/D7965