add enum containing all successful route returns (#614)

* add enum containing all successful route returns

This enum derives `Responder`, so it can be used as route result.
We also implement `From`, so it can be converted

* disable clippy warning about this enum

There's no use trying to chase this warning.
The next warning will be about `Redirect`, which itself is 360 byte,
according to @fdb-hiroshima (thanks for the research!)

So, if anything, we can try to get Rocket to work on that!

* refactor routes/posts to only use one level of Result

* admin settings error is not an ErrorPage, so dont use Result<>

* refactor: use early return to remove indent of main logic

This diff is absolutely atrocious.
but the resulting readability is worth it

* refactor routes/post for early returns & RespondOrRedirect

* refactor routes/session for early returns & use RespondOrRedirect

* refactor routes/user -- add another field to enum

* refactor routes/blogs for early returns & RespondOrRedirect

* refactor routes/blogs for early returns & RespondOrRedirect

This is a final refactor of `pub fn update()` for readability.
We're removing at least one indentation level.

src/routes/mod.rs

@@ -7,15 +7,51 @@ use rocket::{
         RawStr, Status,
     },
     request::{self, FromFormValue, FromRequest, Request},
-    response::NamedFile,
+    response::{Flash, NamedFile, Redirect},
     Outcome,
 };
 use std::path::{Path, PathBuf};
+use template_utils::Ructe;
 
 use plume_models::{posts::Post, Connection};
 
 const ITEMS_PER_PAGE: i32 = 12;
 
+/// Special return type used for routes that "cannot fail", and instead
+/// `Redirect`, or `Flash<Redirect>`, when we cannot deliver a `Ructe` Response
+#[allow(clippy::large_enum_variant)]
+#[derive(Responder)]
+pub enum RespondOrRedirect {
+    Response(Ructe),
+    FlashResponse(Flash<Ructe>),
+    Redirect(Redirect),
+    FlashRedirect(Flash<Redirect>),
+}
+
+impl From<Ructe> for RespondOrRedirect {
+    fn from(response: Ructe) -> Self {
+        RespondOrRedirect::Response(response)
+    }
+}
+
+impl From<Flash<Ructe>> for RespondOrRedirect {
+    fn from(response: Flash<Ructe>) -> Self {
+        RespondOrRedirect::FlashResponse(response)
+    }
+}
+
+impl From<Redirect> for RespondOrRedirect {
+    fn from(redirect: Redirect) -> Self {
+        RespondOrRedirect::Redirect(redirect)
+    }
+}
+
+impl From<Flash<Redirect>> for RespondOrRedirect {
+    fn from(redirect: Flash<Redirect>) -> Self {
+        RespondOrRedirect::FlashRedirect(redirect)
+    }
+}
+
 #[derive(Shrinkwrap, Copy, Clone, UriDisplayQuery)]
 pub struct Page(i32);