Only parse details fields that are requested

At the moment, we are putting the entire contents of the `details` field
through the `DetailsPresenter`, which is sub-optimal as we are parsing
values that clients are not requesting.

By using lookaheads, we can determine which fields from `details` have
been requested and only parse those.

In a previous prototype, we had only transformed the govspeak in the
`body` field. However the schemas can permit mixed govspeak/HTML content
in any details field. We therefore need to parse all items within
`details`, as is already done using the `DetailsPresenter`.

Note: the code here could've been simpler (i.e. not duplicate the
object, just slice the details hash) but the `ContentEmbedPresenter`
requires a full `Edition` object.

app/graphql/types/edition_type.rb

@@ -162,7 +162,7 @@ class WhipOrganisation < Types::BaseObject
     field :content_id, ID
     field :current, Boolean
     field :description, String
-    field :details, Details
+    field :details, Details, extras: [:lookahead]
     field :document_type, String
     field :ended_on, GraphQL::Types::ISO8601DateTime
     field :first_published_at, GraphQL::Types::ISO8601DateTime, null: false
@@ -191,12 +191,16 @@ def change_history
       Presenters::ChangeHistoryPresenter.new(object).change_history
     end
 
-    def details
+    def details(lookahead:)
+      requested_details_fields = lookahead.selections.map(&:name) - [:change_history]
+      object_with_only_requested_details = object.dup
+      object_with_only_requested_details.details = object_with_only_requested_details.details.slice(*requested_details_fields)
+
       Presenters::ContentTypeResolver.new("text/html").resolve(
         Presenters::DetailsPresenter.new(
-          object.details,
+          object_with_only_requested_details.details,
           nil,
-          Presenters::ContentEmbedPresenter.new(object),
+          Presenters::ContentEmbedPresenter.new(object_with_only_requested_details),
         ).details,
       )
     end