Add `Delete` button to label edit view

...sets/javascripts/labels/components/delete_label_modal.vue

app/assets/javascripts/pages/admin/labels/edit/index.js

app/assets/javascripts/pages/groups/labels/edit/index.js

app/assets/javascripts/pages/projects/labels/edit/index.js

app/models/label.rb

app/presenters/label_presenter.rb

app/views/shared/labels/_form.html.haml

doc/user/admin_area/img/admin_labels.png

doc/user/admin_area/img/admin_labels_v14_7.png

doc/user/admin_area/labels.md

doc/user/project/labels.md

spec/factories/labels.rb

spec/features/admin/admin_labels_spec.rb

spec/features/groups/labels/edit_spec.rb

spec/features/projects/labels/user_edits_labels_spec.rb

spec/frontend/labels/delete_label_modal_spec.js

spec/presenters/label_presenter_spec.rb