Logs UI in Server Admin (#374)

* add in ui

* add in logging viewer

* Revert "add in ui"

This reverts commit 9614721fa8a439f7097adca69772b5d41f6585d6.

* finish basic feature

* clean up

* improve and fix build

* add in debug log level command option

* use paging for log file list, use extension to select log files, show message for setting up logging

* make paging a little better

* add very basic UT for logs

* Update ServerController.cs

BTCPayServer/Models/ServerViewModels/LogsViewModel.cs

@@ -0,0 +1,19 @@
+using System.Collections.Generic;
+using System.IO;
+
+namespace BTCPayServer.Models.ServerViewModels
+{
+    public class LogsViewModel
+    {
+        
+        public string StatusMessage
+        {
+            get; set;
+        }
+
+        public List<FileInfo> LogFiles { get; set; } = new List<FileInfo>();
+        public string Log { get; set; }
+        public int LogFileCount { get; set; }
+        public int LogFileOffset{ get; set; }
+    }
+}