Login workflow (#137)

* Use collection label instead of name on the CollectionPage

* Added Avatar and logout menu item

* [feat](login) Added userpic with a logout action in the dropdown.

- Display logged in user in the AppHeader
- Implemented logout action and store + tests
- Better styles for GitHub sign in screen

Closes #100

* Better styles for the AppHeader

src/backends/backend.js

@@ -16,6 +16,10 @@ class LocalStorageAuthStore {
   store(userData) {
     window.localStorage.setItem(this.storageKey, JSON.stringify(userData));
   }
+
+  logout() {
+    window.localStorage.removeItem(this.storageKey);
+  }
 }
 
 const slugFormatter = (template, entryData) => {
@@ -67,6 +71,14 @@ class Backend {
     });
   }
 
+  logout() {
+    if (this.authStore) {
+      this.authStore.logout();
+    } else {
+      throw new Error('User isn\'t authenticated.');
+    }
+  }
+
   listEntries(collection) {
     const collectionModel = new Collection(collection);
     const listMethod = this.implementation[collectionModel.listMethod()];