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

### 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

vector-im/element-meta#1138, vector-im/element-web#2084 and possibly also vector-im/element-web#1857