Enhance files actions to accept arrays of fileids (#2735)

* Enhanced Files action by modifying it to accept a list of SelectedFileIds

* Added checks to verify all files passed to the files action exist, Updated tests

* Enhanced Files action to accept an array of fileIds

* Removed redundant fileId list

BTCPayServer/Models/ServerViewModels/ViewFilesViewModel.cs

@@ -6,8 +6,7 @@ namespace BTCPayServer.Models.ServerViewModels
     public class ViewFilesViewModel
     {
         public List<StoredFile> Files { get; set; }
-        public string DirectFileUrl { get; set; }
-        public string SelectedFileId { get; set; }
+        public Dictionary<string, string> DirectUrlByFiles { get; set; }
         public bool StorageConfigured { get; set; }
     }
 }