feat(db): Add view for ordering thread index by most recent post

This implements the classic thread ordering expected by most forum
users (i.e. the thread with the most recent post is at the top).

src/models.rs

@@ -26,6 +26,18 @@ pub struct Thread {
     pub author_email: String,
 }
 
+/// This struct is used as the query type for the thread index view,
+/// which lists the index of threads ordered by the last post in each
+/// thread.
+#[derive(Queryable, Serialize)]
+pub struct ThreadIndex {
+    pub thread_id: i32,
+    pub title: String,
+    pub author_name: String,
+    pub posted: DateTime<Utc>,
+    pub post_id: i32,
+}
+
 #[derive(Identifiable, Queryable, Serialize, Associations)]
 #[belongs_to(Thread)]
 pub struct Post {