Consider making the sender generate url previews, as with e2e thumbnails

[ghost]

Description

Consider making the sender generate url previews. We are already trusting clients to faithfully generate thumbnails in e2e encrypted rooms anyway, so this is not a big jump in trust. Furthermore, someone who posts a link to malicious content under their control can already make the url preview say whatever they want, regardless of whether it is generated by a homeserver or sent directly.

On the plus side, it improves privacy, especially for single-user homeservers. It also improves
efficiency, as the site is only accessed once, rather than by every homeserver in the room, and reduces load on the servers.

Steps to reproduce

• n/a

Log: not sent

Version information

n/a

• Platform: any

For the web app:

• Browser: any
• OS: any
• URL: any

For the desktop app:

• OS: any
• Version: future

Related

#1138, element-hq/element-web#2084 and possibly also element-hq/element-web#1857

[ara4n]

yeah, i'm inclined to agree, although i think we should still let clients have the option of generating their own thumbs if they prefer.

we clearly need a standard API for clientside thumbnailing in general, which links in turn into the whole general "human representation of events" problem.

[ara4n]

This also has the advantage of letting the sender sculpt what the preview looks like in future (as you can do on FB for instance)