Apply ref_option lint findings (#7143)

Quote from the lint description:
"More flexibility, better memory optimization, and more idiomatic Rust code.

&Option<T> in a function signature breaks encapsulation because the caller must own T and move it into an Option to call with it. When returned, the owner must internally store it as Option<T> in order to return it. At a lower level, &Option<T> points to memory with the presence bit flag plus the T value, whereas Option<&T> is usually optimized to a single pointer, so it may be more optimal."

src/api/core/mod.rs

@@ -124,7 +124,7 @@ async fn post_eq_domains(data: Json<EquivDomainData>, headers: Headers, conn: Db
 
     user.save(&conn).await?;
 
-    nt.send_user_update(UpdateType::SyncSettings, &user, &headers.device.push_uuid, &conn).await;
+    nt.send_user_update(UpdateType::SyncSettings, &user, headers.device.push_uuid.as_ref(), &conn).await;
 
     Ok(Json(json!({})))
 }