Add custom_details and custom_activity_details as first routes

how this works: we use find_by_host() to find the Host in question, the
defer to the existing function!

Caveat: Currently, we, in that function, we do another lookup DB lookup
for the Blog, even thou we already know it.
It might be wise, to have both of those another wrapper here?!

plume-models/src/blogs.rs

@@ -195,6 +195,23 @@ impl Blog {
         }
     }
 
+    pub fn find_by_host(c: &PlumeRocket, host: Host) -> Result<Blog> {
+        let from_db = blogs::table
+            .filter(blogs::custom_domain.eq(host))
+            .limit(1)
+            .load::<Blog>(&*c.conn)?
+            .into_iter()
+            .next();
+        if let Some(from_db) = from_db {
+            Ok(from_db)
+        } else {
+            // we should never get here, because
+            // a) load::<Blog>()? should return early if it fails
+            // b) this function is only called after a Request::guard::<Host>()
+            Err(Error::NotFound)
+        }
+    }
+
     fn fetch_from_webfinger(c: &PlumeRocket, acct: &str) -> Result<Blog> {
         resolve_with_prefix(Prefix::Group, acct.to_owned(), true)?
             .links