Post activities to the correct inbox

Don't assume everyone is following Plume's routing

src/models/blogs.rs

@@ -126,6 +126,10 @@ impl Actor for Blog {
         ActorType::Blog
     }
 
+    fn get_inbox_url(&self) -> String {
+        self.inbox_url.clone()
+    }
+
     fn from_url(conn: &PgConnection, url: String) -> Option<Blog> {
         blogs::table.filter(blogs::ap_url.eq(url))
             .limit(1)

src/models/users.rs

@@ -263,6 +263,10 @@ impl Actor for User {
         ActorType::Person
     }
 
+    fn get_inbox_url(&self) -> String {
+        self.inbox_url.clone()
+    }
+
     fn custom_props(&self, conn: &PgConnection) -> serde_json::Map<String, serde_json::Value> {
         let mut res = serde_json::Map::new();
         res.insert("publicKey".to_string(), json!({