Enable multi-page history for back button

Previously, the back button just navigated to the home page. Now, it
can navigate back like one would expect.

src/containers/PagedApp.js

@@ -10,7 +10,7 @@ const mapStateToProps = state => {
   return {
     selectedGoal: goal,
     title: state.page.selectedGoalId === null ? 'Subgoals' : goal.name,
-    hasBack: state.page.selectedGoalId != null
+    hasBack: state.page.history.length > 0
   };
 };
 

src/ducks/page.js

@@ -4,21 +4,23 @@ const GO_BACK = 'subgoals/page/GO_BACK';
 
 // Reducer
 const INITIAL_STATE = {
-  selectedGoalId: null
+  selectedGoalId: null,
+  history: []
 };
 
 export default function reducer(state = INITIAL_STATE, action) {
   switch (action.type) {
     case SET_SELECTED_GOAL:
       return {
         ...state,
-        selectedGoalId: action.goalId
+        selectedGoalId: action.goalId,
+        history: [...state.history, state.selectedGoalId]
       };
 
     case GO_BACK:
       return {
         ...state,
-        selectedGoalId: null
+        selectedGoalId: state.history.pop()
       }
     default: return state;
   }