Refactor DropdownContentsEditView component to use GraphQL + Apollo

Currently, app/assets/javascripts/vue_shared/components/sidebar/labels_select_vue/dropdown_contents_labels_view.vue has a few Vuex bindings including the one to fetch labels. In the scope of &6092 (closed), we need to replace REST API with GraphQL, pass data we get from Vuex as props, and emit events for toggling the dropdown.

labels, receiveLabelsSuccess, fetchLabels and labelsFetchInProgress Vuex bindings can be removed during this refactoring