Redirect on invalid UTF-8 in /p/

Previously, uploading a paste with invalid UTF-8 and then viewing it
with the pretty URL would cause a panic.
With this change, it simply redirects to the raw URL.

src/models/mod.rs

@@ -1,3 +1,4 @@
+pub mod maybe_redirect;
 pub mod paste_id;
 pub mod pretty;
 pub mod pretty_syntax;