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/schema.rs

@@ -13,7 +13,6 @@ table! {
     threads (id) {
         id -> Int4,
         title -> Varchar,
-        body -> Text,
         posted -> Timestamptz,
         author_name -> Varchar,
         author_email -> Varchar,