overhaul using a trip struct that notifies its ui dependencies

frontend/lib/modules/landmark_card.dart

@@ -1,4 +1,5 @@
 import 'package:anyway/structs/landmark.dart';
+import 'package:cached_network_image/cached_network_image.dart';
 import 'package:flutter/material.dart';
 
 
@@ -31,9 +32,10 @@ class _LandmarkCardState extends State<LandmarkCard> {
               height: double.infinity,
               // force a fixed width
               width: 160,
-              child: Image.network(
-                widget.landmark.imageURL ?? '',
-                errorBuilder: (context, error, stackTrace) => Icon(Icons.question_mark_outlined),
+              child: CachedNetworkImage(
+                imageUrl: widget.landmark.imageURL ?? '',
+                placeholder: (context, url) => CircularProgressIndicator(),
+                errorWidget: (context, error, stackTrace) => Icon(Icons.question_mark_outlined),
                 // TODO: make this a switch statement to load a placeholder if null
                 // cover the whole container meaning the image will be cropped
                 fit: BoxFit.cover,