create & attach an AdHoc Fairing for dealing with Custom Domains

this rewrites the URL to /custom_domain/<url>
(we have no route handlers for this path yet)

Lots of help from @fdb-hiroshima & @BaptisteGelez in dealing with the
borrow checker — thank you 💜️

plume-models/src/blogs.rs

@@ -25,6 +25,7 @@ use posts::Post;
 use safe_string::SafeString;
 use schema::blogs;
 use search::Searcher;
+use std::fmt::{self, Display};
 use users::User;
 use {Connection, Error, PlumeRocket, Result};
 
@@ -39,6 +40,12 @@ impl Host {
     }
 }
 
+impl Display for Host {
+    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
+        write!(f, "{}", self.0)
+    }
+}
+
 impl AsRef<str> for Host {
     fn as_ref(&self) -> &str {
         &self.0