[web] Fix thread gallery for encrypted media

Summary:
This diff fixes ENG-3946.
Added proper source mapping depending on media type and replaced <img> with GalleryItem component added in D7994.

Also fixed filtering condition for encrypted media.

Depends on D7994

Test Plan: Open media gallery for encrypted and non-encrypted media. Check that all images are displayed properly. Throttle network, then check thumbhashes are displayed while images are loading.

Reviewers: inka, ginsu, atul, kuba

Reviewed By: atul

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D7995