Clear active collection if navigating out of it directly

Fixes #474

app/scenes/Collection/Collection.js

@@ -49,6 +49,10 @@ class CollectionScene extends Component {
     }
   }
 
+  componentWillUnmount() {
+    this.props.ui.clearActiveCollection();
+  }
+
   loadContent = async (id: string) => {
     const { collections } = this.props;
 

app/scenes/Document/Document.js

@@ -63,7 +63,7 @@ class DocumentScene extends Component {
   @observable notFound = false;
   @observable moveModalOpen: boolean = false;
 
-  componentWillMount() {
+  componentDidMount() {
     this.loadDocument(this.props);
   }
 

app/stores/UiStore.js

@@ -35,6 +35,11 @@ class UiStore {
     this.activeCollectionId = collection.id;
   };
 
+  @action
+  clearActiveCollection = (): void => {
+    this.activeCollectionId = undefined;
+  };
+
   @action
   clearActiveDocument = (): void => {
     this.activeDocumentId = undefined;