fix: Resolve unnecessary serialization issue

Fixes #1335

changelog.d/1340.bugfix

@@ -0,0 +1 @@
+Use exists instead of contains to save writing to a buffer in src/service/users/mod.rs: is_login_disabled. Contributed by @aprilgrimoire.

src/service/users/mod.rs

@@ -304,7 +304,11 @@ impl Service {
 	pub fn enable_login(&self, user_id: &UserId) { self.db.userid_logindisabled.remove(user_id); }
 
 	pub async fn is_login_disabled(&self, user_id: &UserId) -> bool {
-		self.db.userid_logindisabled.contains(user_id).await
+		self.db
+			.userid_logindisabled
+			.exists(user_id.as_str())
+			.await
+			.is_ok()
 	}
 
 	/// Check if account is active, infallible