improved labelling system style and feel

core/migrations/0016_auto_20190521_1803.py

core/models/feedback.py

frontend/src/components/feedback_labels/FeedbackLabel.vue

...tend/src/components/feedback_labels/FeedbackLabelForm.vue

...d/src/components/feedback_labels/FeedbackLabelUpdater.vue

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

frontend/src/components/feedback_labels/LabelSelector.vue

frontend/src/components/mixins/commentLabelSelector.ts

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

.../src/components/submission_notes/base/FeedbackComment.vue

frontend/src/models.ts

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