refactor(db): Store thread body in the posts table

This is a simplification over the previous approach. The OP of a
thread is just a normal post like any other in this model, which
allows some code simplifications (and future query convenience).

src/models.rs

@@ -5,7 +5,6 @@ use schema::{threads, posts};
 pub struct Thread {
     pub id: i32,
     pub title: String,
-    pub body: String,
     pub posted: DateTime<Utc>,
     pub author_name: String,
     pub author_email: String,
@@ -26,7 +25,6 @@ pub struct Post {
 #[table_name="threads"]
 pub struct NewThread {
     pub title: String,
-    pub body: String,
     pub author_name: String,
     pub author_email: String,
 }