Allow 'Hidden by default' media previews for rooms that may contain sensitive/NSFW media

[Kelketek]

Is your suggestion related to a problem? Please describe.
As a user, I would like to be able to communicate with my friends sharing NSFW content in a chat while in public, without the fear that NSFW media will be visible to anyone who glimpses over my shoulder.

Describe the solution you'd like
Allow for a toggle, similar to the URL preview toggles, that enables 'tap to display' on media. When tapped, it should load the media and preview it. Potentially, the file might not be downloaded at all until tapped, or it may be downloaded but held in reserve without preview, depending on implementer preference.

Describe alternatives you've considered
A global toggle that instantly hides or displays all media in a chat room, with placeholder blocks that say when a piece of media has been shared, with a reminder of how to toggle.

[Half-Shot]

This exists as "Show previews/thumbnails for images" in settings globally, and the ability to specify per-room is being looked at under matrix-org/matrix-spec-proposals#4278 .

Additionally, you can now hide all media individually by clicking the hide button.