Have link previews of posts be specific to them

**Is your feature request related to a problem? Please describe.**
When I share a post with friends on WhatsApp (and other places I guess), the link preview is generic, which is weird and also doesn't allow to quickly see what the post is about.


**Describe the solution you'd like**
That the link would show the specific info of the post (if it is publicly accessible, of course)

**Describe alternatives you've considered**
Sharing the link to the same post on another instance?

**Additional context**
I would love if the link preview image inherited from the post's media attachment :)

No files or tests are named in the issue. Start by locating how publicly accessible post links are rendered and how the app supplies link-preview metadata; done means a shared post shows its specific information and, when available, its media attachment as the preview image.

Written by the indexing model from the issue text.