moving the lisp presentation parsing to service_items

Since we need the nav_bar to have the ServiceItemModel, the
presentation needs to be a list of service items, not slides, each
service item will have a list of slides attached though.

src/core/slide.rs

@@ -228,6 +228,10 @@ impl Slide {
 
         Ok(slides)
     }
+
+    // pub fn slides_from_item(item: &ServiceItem) -> Result<Vec<Self>> {
+    //     todo!()
+    // }
 }
 
 impl From<Value> for Slide {