Add a preparing state before any other MR states

..._request_widget/components/states/mr_widget_preparing.vue

app/assets/javascripts/vue_merge_request_widget/constants.js

app/assets/javascripts/vue_merge_request_widget/i18n.js

...avascripts/vue_merge_request_widget/mr_widget_options.vue

...rge_request_widget/queries/get_state.subscription.graphql

...ascripts/vue_merge_request_widget/stores/get_state_key.js

...cripts/vue_merge_request_widget/stores/mr_widget_store.js

...javascripts/vue_merge_request_widget/stores/state_maps.js

locale/gitlab.pot

...uest_widget/components/states/mr_widget_preparing_spec.js

...ontend/vue_merge_request_widget/mr_widget_options_spec.js

...end/vue_merge_request_widget/stores/get_state_key_spec.js