Add button to compact realm on demand

In general we're doing things correctly so the realm file shouldn't
expand (unless mass deletions are made from it), but this is a nice way
to manually confirm the behaviour.

Sometimes if using realm studio with osu! running, for instance, the
realm file size can blow out of proportion. This will recover from such
cases.

Note that calling `RealmFactory.Compact` itself is not enough, as it
will fail unless all instances of the realm have been closed.

osu.Game/Localisation/DebugSettingsStrings.cs

@@ -44,6 +44,11 @@ namespace osu.Game.Localisation
         /// </summary>
         public static LocalisableString ClearAllCaches => new TranslatableString(getKey(@"clear_all_caches"), @"Clear all caches");
 
+        /// <summary>
+        /// "Compact realm"
+        /// </summary>
+        public static LocalisableString CompactRealm => new TranslatableString(getKey(@"compact_realm"), @"Compact realm");
+
         private static string getKey(string key) => $"{prefix}:{key}";
     }
 }