Move beatmap collections to realm

[peppy]

Sorry for the size of this diff. Collections were written in a very convoluted way and I ended up having to touch basically everything they touched. I'd recommend ignoring the diff where it's overwhelming and checking the files as if they are new implementations.

• To simplify, I've removed a lot of bindable flow. This means that some UI elements are doing full refreshes rather than partial updates (ie. when a collection is renamed). We can add such things back if required, but it doesn't feel too bad for now, even with tens of collections. And it's a rare operation.
• Still using hashes, reasoning is mentioned inline https://github.com/ppy/osu/compare/master...peppy:osu:realm-collections?expand=1#diff-862af16e9f3547129791348268ee0b26e35bf8b8df70999b03cc9b0b3e354b6eR29-R36. I'm open to migrating to beatmap references in a future if that's seen as something we need.
• Realm version bump isn't actually required for adding new models, but I did bump to (ab)use the migration flow to handle migrating away from the local collection.db.
• Fixed some weird edge cases along the way, such as song select re-filtering when clicking the "Manage collections" item in the filter dropdown.
• The structure of the dropdown and its menu items was pretty bad, and still is. I think this needs a re-think at the framework side to resolve.
• I've left the "manage collections" dialog allowing ordering, but this isn't currently saved out. I figure people will complain if they have been actually using the rearrangeability, else we can consider removing it in the future.
  This is a somewhat necessary precursor to having the import-update flow handle collections properly.

[smoogipoo]

Right clicking on playlist panels in multiplayer crashes with:

System.NotSupportedException: The method 'Cast' is not supported
   at Realms.RealmResultsVisitor.VisitMethodCall(MethodCallExpression node)
   at Realms.RealmResults`1.GetOrCreateHandle()
   at System.Lazy`1.ViaFactory(LazyThreadSafetyMode mode)
   at System.Lazy`1.ExecutionAndPublication(LazyHelper executionAndPublication, Boolean useDefaultConstructor)
   at System.Lazy`1.CreateValue()
   at Realms.RealmCollectionBase`1.get_Count()
   at System.Collections.Generic.List`1..ctor(IEnumerable`1 collection)
   at System.Linq.Enumerable.ToList[TSource](IEnumerable`1 source)
   at osu.Game.Screens.OnlinePlay.DrawableRoomPlaylistItem.get_ContextMenuItems() in /home/smgi/Repos/osu/osu.Game/Screens/OnlinePlay/DrawableRoomPlaylistItem.cs:line 502
   at osu.Framework.Graphics.Cursor.ContextMenuContainer.<>c.<OnMouseDown>b__9_0(IHasContextMenu t)
   at System.Linq.Enumerable.SelectListIterator`2.MoveNext()
   at System.Linq.Enumerable.TryGetFirst[TSource](IEnumerable`1 source, Func`2 predicate, Boolean& found)
   at System.Linq.Enumerable.FirstOrDefault[TSource](IEnumerable`1 source, Func`2 predicate)
   at osu.Framework.Graphics.Cursor.ContextMenuContainer.OnMouseDown(MouseDownEvent e)
   at System.Linq.Enumerable.TryGetFirst[TSource](IEnumerable`1 source, Func`2 predicate, Boolean& found)

[peppy]

Right clicking on playlist panels in multiplayer crashes with:

Should be fixed. A case of copy-paste implementations (not new in this PR) and only applying a fix in one of two places 😓 .