Add extension methods to add extra safety to realm subscriptions

[peppy]

Also adjusts the naming and documentation to make it (hopefully) easier to understand what this method/process implies.

Diff for testing banned symbols:

diff --git a/osu.Game.Tests/Database/RealmLiveTests.cs b/osu.Game.Tests/Database/RealmLiveTests.cs
index 9b6769b788..6585d2e90d 100644
--- a/osu.Game.Tests/Database/RealmLiveTests.cs
+++ b/osu.Game.Tests/Database/RealmLiveTests.cs
@@ -2,6 +2,7 @@
 // See the LICENCE file in the repository root for full licence text.
 
 using System;
+using System.Collections.Generic;
 using System.Diagnostics;
 using System.Linq;
 using System.Threading.Tasks;
@@ -208,7 +209,13 @@ public void TestLiveAssumptions()
 
                 using (var updateThreadContext = realmFactory.CreateContext())
                 {
-                    updateThreadContext.All<RealmBeatmap>().QueryAsyncWithNotifications(gotChange);
+                    IList<RealmBeatmap> testList = updateThreadContext.All<RealmBeatmap>().ToList();
+                    IQueryable<RealmBeatmap> testQueryable = updateThreadContext.All<RealmBeatmap>().AsQueryable();
+
+                    testList.SubscribeForNotifications(gotChange);
+                    testQueryable.SubscribeForNotifications(gotChange);
+
+                    updateThreadContext.All<RealmBeatmap>().SubscribeForNotifications(gotChange);
                     ILive<RealmBeatmap>? liveBeatmap = null;
 
                     Task.Factory.StartNew(() =>