Refactor `DropdownContentsCreateView` component to use GraphQL + Apollo

Currently, app/assets/javascripts/vue_shared/components/sidebar/labels_select_vue/dropdown_contents_create_view.vue has a few Vuex bindings and performs a REST API call to create a new label. 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.

labelCreateInProgress and createLabel Vuex bindings can be removed during this refactoring,