New: Added support for Gotify notifications (#3474)

* Added support for Gotify notifications

* Use string interpolation to build url

* Adapt changes by markus101 over at Sonarr

* Remove blank line

Fixes #3472

src/NzbDrone.Core/Notifications/Gotify/InvalidResponseException.cs

@@ -0,0 +1,15 @@
+using System;
+
+namespace NzbDrone.Core.Notifications.Gotify
+{
+    public class InvalidResponseException : Exception
+    {
+        public InvalidResponseException()
+        {
+        }
+
+        public InvalidResponseException(string message) : base(message)
+        {
+        }
+    }
+}