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/maybe_redirect.rs

@@ -0,0 +1,20 @@
+use rocket::{response::Redirect, Responder};
+use rocket_dyn_templates::Template;
+
+#[derive(Responder)]
+pub enum MaybeRedirect {
+    Redirect(Redirect),
+    Template(Template),
+}
+
+impl From<Redirect> for MaybeRedirect {
+    fn from(other: Redirect) -> Self {
+        Self::Redirect(other)
+    }
+}
+
+impl From<Template> for MaybeRedirect {
+    fn from(other: Template) -> Self {
+        Self::Template(other)
+    }
+}

src/models/mod.rs

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