feat(main): Add support for Tera templates

Sets up the structure required to carry Tera templates in the
actix-web state.

The (still very barebones) index renderer has been updated to render a
Tera template.

src/models.rs

@@ -1,6 +1,6 @@
 use chrono::prelude::{DateTime, Utc};
 
-#[derive(Queryable)]
+#[derive(Queryable, Serialize)]
 pub struct Thread {
     pub id: i32,
     pub title: String,