Fix ModelManager not correctly re-retrieving realm objects before performing operations

Falls into the age-old trap of attempting to retrieve an item from realm
without first ensuring that realm is in an up-to-date state.

Consider this scenario:

- Editor is entered from main menu, causing it to create a new beatmap
  from its async `load()` method.
- Editor opens correctly, then main thread performs a file operations on
  the same beatmap.
- Main thread is potentially not refreshed yet, and will result in `null`
  instance when performing the re-fetch in `performFileOperation`.

I've fixed this by using the safe implementation inside `RealmLive<T>`.
Feels like we want this is one place which is always used as the correct
method.

On a quick search, there are 10-20 other usages of `Realm.Find<T>` which
could also have similar issues, but it'll be a bit of a pain to go
through and fix each of these. In 99.9% of cases, the accesses are on
instances which couldn't have just been created (or the usage of
recently-imported/created is blocked by realm subscription flows, ie.
baetmap import) so I'm not touching them for now.

Something to keep in mind when working with realm going forward though.

osu.Game/Database/ModelManager.cs

@@ -48,16 +48,13 @@ namespace osu.Game.Database
             // This method should be removed as soon as all the surrounding pieces support non-detached operations.
             if (!item.IsManaged)
             {
-                // Importantly, begin the realm write *before* re-fetching, else the update realm may not be in a consistent state
-                // (ie. if an async import finished very recently).
-                Realm.Realm.Write(realm =>
+                // We use RealmLive here as it handled re-retrieval and refreshing of realm if required.
+                new RealmLive<TModel>(item.ID, Realm).PerformWrite(i =>
                 {
-                    var managed = realm.Find<TModel>(item.ID);
-                    Debug.Assert(managed != null);
-                    operation(managed);
+                    operation(i);
 
                     item.Files.Clear();
-                    item.Files.AddRange(managed.Files.Detach());
+                    item.Files.AddRange(i.Files.Detach());
                 });
             }
             else