Adding an aggregate torznab feed for all configured trackers (#1312)

* Adding an aggregate torznab feed for all configured trackers

This adds just a basic first implementation of a special
torznab feed that will make all configured trackers available
with all the supported query params.
- This should close #1247 
- This also contributes to #921 

* Adding missing file

* Add missing implementation from Test project

src/Jackett.Test/TestIIndexerManagerServiceHelper.cs

@@ -52,5 +52,10 @@ namespace JackettTest
         {
             throw new NotImplementedException();
         }
+
+        public void InitAggregateIndexer()
+        {
+            throw new NotImplementedException();
+        }
     }
 }