added labelling system components and vuex modules

core/migrations/0015_feedbacklabel_colour.py

core/models/label.py

core/serializers/label.py

frontend/src/api.ts

frontend/src/components/feedback_labels/FeedbackLabel.vue

...end/src/components/feedback_labels/FeedbackLabelsList.vue

frontend/src/components/feedback_labels/LabelSelector.vue

.../src/components/submission_notes/SubmissionCorrection.vue

...ponents/submission_notes/base/BaseAnnotatedSubmission.vue

...tend/src/components/submission_notes/base/CommentForm.vue

...s/submission_notes/toolbars/AnnotatedSubmissionLabels.vue

frontend/src/models.ts

frontend/src/pages/base/TutorReviewerBaseLayout.vue

frontend/src/store/modules/feedback-labels.ts

frontend/src/store/modules/submission-notes.ts

frontend/src/store/store.ts