Add support for quoted posts

This is starting to become a common use case, and despite Mastodon not implementing it (yet) Tokodon can implement it client-side.

How it works is that Tokodon looks through all the URLs of a post, and then figures out if it's a valid post or not. To prevent hammering the server, a basic check is performed to see if it looks like a typical post URL. Then it's fetched from the server and displayed inline on the timeline.