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 +1,31 @@
+use rocket::{
+    request::Request,
+    response::{Redirect, Responder, Result},
+};
+use rocket_dyn_templates::Template;
+
+pub enum MaybeRedirect {
+    Redirect(Box<Redirect>),
+    Template(Box<Template>),
+}
+
+impl From<Redirect> for MaybeRedirect {
+    fn from(other: Redirect) -> Self {
+        Self::Redirect(Box::new(other))
+    }
+}
+
+impl From<Template> for MaybeRedirect {
+    fn from(other: Template) -> Self {
+        Self::Template(Box::new(other))
+    }
+}
+
+impl<'r, 'o: 'r> Responder<'r, 'o> for MaybeRedirect {
+    fn respond_to(self, req: &'r Request<'_>) -> Result<'o> {
+        match self {
+            Self::Template(t) => t.respond_to(req),
+            Self::Redirect(r) => r.respond_to(req),
+        }
+    }
+}