Implemented a half-way okay-ish user interaction pattern: mouse drag.

src/reducer.js

@@ -0,0 +1,48 @@
+import handleUserClicked from "./userpathhandler";
+
+export default function reduce(state, action) {
+    switch (action.type) {
+        case 'started_loading': {
+            return {
+                ...state,
+                maze: null,
+                loading: true,
+                errorMessage: null
+            }
+        }
+        case 'loaded_maze': {
+            return {
+                ...state,
+                loading: false,
+                maze: action.maze,
+                userPath: []
+            }
+        }
+        case 'loading_failed': {
+            return {
+                ...state,
+                loading: false,
+                errorMessage: `Failed to load maze. Reason: ${action.reason}`
+            }
+        }
+        case 'toggled_show_solution': {
+            return {
+                ...state,
+                showSolution: action.value
+            }
+        }
+        case 'closed_message_banner': {
+            return {
+                ...state,
+                errorMessage: null
+            }
+        }
+        case 'clicked_cell': {
+            // There's so much logic involved, externalize that into its own file.
+            return handleUserClicked(state, action.x, action.y);
+        }
+        default: {
+            throw new Error(`Unknown action: ${action.type}`);
+        }
+    }
+}