Fix challenge sort order to match the server-side ordering. (#1330)

The app was sorting the challenges by decreasing member count, but the server
returns challenges in order of creation date (with official challenges overriding
the ordering). This should fix the challenges jumping around.

Fixes #1307.

Habitica Database/Habitica Database/Repositories/SocialLocalRepository.swift

@@ -310,7 +310,7 @@ public class SocialLocalRepository: BaseLocalRepository {
             query = RealmChallenge.findAll()
         }
         // swiftlint:disable:next force_unwrapping
-        return query!.sorted(key: "memberCount", ascending: false).reactive().map({ (value, changeset) -> ReactiveResults<[ChallengeProtocol]> in
+        return query!.sorted(by: [SortDescriptor(keyPath: "official", ascending: false), SortDescriptor(keyPath: "createdAt", ascending: false)]).reactive().map({ (value, changeset) -> ReactiveResults<[ChallengeProtocol]> in
             return (value.map({ (challenge) -> ChallengeProtocol in return challenge }), changeset)
         })
     }