add multi collections (#315)

* start to add multi-collections

* create multi collection with no items

* edit multi collections

* fix plex credentials threading issue

* add playback order to multi collection items

* group episodes outside of shuffled enumerator

* move playback order onto each schedule item

* fix multi collection grouping

* update changelog

ErsatzTV.Core.Tests/Fakes/FakeMediaCollectionRepository.cs

@@ -4,6 +4,7 @@ using System.Linq;
 using System.Threading.Tasks;
 using ErsatzTV.Core.Domain;
 using ErsatzTV.Core.Interfaces.Repositories;
+using ErsatzTV.Core.Scheduling;
 using LanguageExt;
 
 namespace ErsatzTV.Core.Tests.Fakes
@@ -18,7 +19,16 @@ namespace ErsatzTV.Core.Tests.Fakes
             throw new NotSupportedException();
 
         public Task<List<MediaItem>> GetItems(int id) => _data[id].ToList().AsTask();
+        public Task<List<MediaItem>> GetMultiCollectionItems(int id) => throw new NotSupportedException();
+
+        public Task<List<CollectionWithItems>> GetMultiCollectionCollections(int id) =>
+            throw new NotSupportedException();
+
         public Task<List<int>> PlayoutIdsUsingCollection(int collectionId) => throw new NotSupportedException();
+
+        public Task<List<int>> PlayoutIdsUsingMultiCollection(int multiCollectionId) =>
+            throw new NotSupportedException();
+
         public Task<bool> IsCustomPlaybackOrder(int collectionId) => false.AsTask();
     }
 }