change post.content and comment.content's types from String to SafeString

src/main.rs

@@ -30,6 +30,7 @@ extern crate serde_derive;
 #[macro_use]
 extern crate serde_json;
 extern crate url;
+extern crate ammonia;
 
 use diesel::{pg::PgConnection, r2d2::{ConnectionManager, Pool}};
 use dotenv::dotenv;