Transaction Support (#309)

* Added transactions to UnitOfWork and refactored code to use it.

* This included blank UI fix from Kavita-webui

API/Interfaces/IUnitOfWork.cs

@@ -11,7 +11,10 @@ namespace API.Interfaces
         ISettingsRepository SettingsRepository { get; }
         IAppUserProgressRepository AppUserProgressRepository { get; }
         ICollectionTagRepository CollectionTagRepository { get; }
-        Task<bool> Complete();
+        bool Commit();
+        Task<bool> CommitAsync();
         bool HasChanges();
+        bool Rollback();
+        Task<bool> RollbackAsync();
     }
 }