[web] Add MediaGalleryItem component

Summary:
Proper displaying of encrypted and non-encrypted thread gallery media required more code so I extracted it into a separate component.
Basically, most of the work is already done by the <EncryptedMultimedia> component so I only needed to add thumbhash and background for non-encrypted images.

Depends on D7993

Test Plan: Tested together with next diff where this is used.

Reviewers: inka, ginsu, atul, kuba

Reviewed By: atul

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D7994