Added LabelStatistics endpoint and Statics overview page in frontend

.gitignore

core/urls.py

core/views/__init__.py

core/views/common_views.py

core/views/label.py

frontend/src/api.ts

frontend/src/components/CorrectionStatistics.vue

frontend/src/components/LabelStatistics.vue

frontend/src/components/student_list/StudentList.vue

frontend/src/components/subscriptions/SubscriptionList.vue

frontend/src/models.ts

frontend/src/pages/Statistics.vue

frontend/src/pages/base/TutorReviewerBaseLayout.vue

frontend/src/router/index.ts

frontend/yarn.lock