web & server: plumb debugging info for content type hierarchy.

Also cleanup Email trait.

web/graphql/show_thread.graphql

@@ -23,9 +23,11 @@ query ShowThreadQuery($threadId: String!) {
         }
         ... on PlainText {
           contents
+          contentTree
         }
         ... on Html {
           contents
+          contentTree
         }
       }
       path