Add support for remote interact (#519)

Add support for remote interaction ([this thing](https://eldritch.cafe/users/Barmaid/remote_follow) in mastodon)

- [x] create the endpoint dispatching remote interactions to local pages
- [x] add this endpoint to web-finger
- [x] propose remote interaction when following and not connected
- [x] propose remote interaction when liking/sharing and not connected
- [x] propose remote interaction when commenting and not connected
- [x] fetch posts/comments we don't know but remote interaction was requested for ?

src/routes/mod.rs

@@ -71,6 +71,12 @@ impl Default for Page {
     }
 }
 
+/// A form for remote interaction, used by multiple routes
+#[derive(Clone, Default, FromForm)]
+pub struct RemoteForm {
+    pub remote: String,
+}
+
 pub fn post_to_atom(post: Post, conn: &Connection) -> Entry {
     EntryBuilder::default()
         .title(format!("<![CDATA[{}]]>", post.title))