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

Also cleanup Email trait.

web/graphql/schema.json

@@ -159,6 +159,22 @@
                   "ofType": null
                 }
               }
+            },
+            {
+              "args": [],
+              "deprecationReason": null,
+              "description": null,
+              "isDeprecated": false,
+              "name": "contentTree",
+              "type": {
+                "kind": "NON_NULL",
+                "name": null,
+                "ofType": {
+                  "kind": "SCALAR",
+                  "name": "String",
+                  "ofType": null
+                }
+              }
             }
           ],
           "inputFields": null,
@@ -400,6 +416,22 @@
                   "ofType": null
                 }
               }
+            },
+            {
+              "args": [],
+              "deprecationReason": null,
+              "description": null,
+              "isDeprecated": false,
+              "name": "contentTree",
+              "type": {
+                "kind": "NON_NULL",
+                "name": null,
+                "ofType": {
+                  "kind": "SCALAR",
+                  "name": "String",
+                  "ofType": null
+                }
+              }
             }
           ],
           "inputFields": null,

web/graphql/show_thread.graphql

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