[web] cleanup thread settings media gallery modal

Summary:
This diff cleans up the one off styles in the thread settings media gallery modal and replaces it with styles of the redesigned modal.

Linear task: https://linear.app/comm/issue/ENG-5943/extendmodify-the-modal-props-api-to-follow-new-modal-designs and https://linear.app/comm/issue/ENG-5944/fix-up-chat-media-modal

Depends on D10323

Test Plan:
Please see screenshots below

no media:

no images:

no videos:

Reviewers: atul, rohan, kamil

Reviewed By: rohan

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D10324