Flash is now a ES6 module

Reduced the technical debt around our JS flash function by making it a
module that is imported rather than relying on the global function.
The global function still exists mainly for technical debt with how
some requests are being completed, but new JS should import the module
directly.

Also reduces some tech debt in the file by removing the need for jQuery.
Instead Flash is now 100% vanilla JS.

app/assets/javascripts/vue_merge_request_widget/components/mr_widget_deployment.js

-/* global Flash */
-
 import '~/lib/utils/datetime_utility';
+import Flash from '../../flash';
 import MemoryUsage from './mr_widget_memory_usage';
 import StatusIcon from './mr_widget_status_icon';
 import MRWidgetService from '../services/mr_widget_service';

app/assets/javascripts/vue_merge_request_widget/components/states/mr_widget_merge_when_pipeline_succeeds.js

-/* global Flash */
+import Flash from '../../../flash';
 import statusIcon from '../mr_widget_status_icon';
 import MRWidgetAuthor from '../../components/mr_widget_author';
 import eventHub from '../../event_hub';

app/assets/javascripts/vue_merge_request_widget/components/states/mr_widget_merged.js

-/* global Flash */
-
+import Flash from '../../../flash';
 import mrWidgetAuthorTime from '../../components/mr_widget_author_time';
 import tooltip from '../../../vue_shared/directives/tooltip';
 import loadingIcon from '../../../vue_shared/components/loading_icon.vue';

app/assets/javascripts/vue_merge_request_widget/components/states/mr_widget_ready_to_merge.js

-/* global Flash */
 import successSvg from 'icons/_icon_status_success.svg';
 import warningSvg from 'icons/_icon_status_warning.svg';
 import simplePoll from '~/lib/utils/simple_poll';
+import Flash from '../../../flash';
 import statusIcon from '../mr_widget_status_icon';
 import eventHub from '../../event_hub';

app/assets/javascripts/vue_merge_request_widget/components/states/mr_widget_wip.js

-/* global Flash */
+import Flash from '../../../flash';
 import statusIcon from '../mr_widget_status_icon';
 import tooltip from '../../../vue_shared/directives/tooltip';
 import eventHub from '../../event_hub';

app/assets/javascripts/vue_merge_request_widget/mr_widget_options.js

-/* global Flash */
-
+import Flash from '../flash';
 import {
   WidgetHeader,
   WidgetMergeHelp,

app/assets/javascripts/vue_shared/components/markdown/field.vue

 <script>
-  /* global Flash */
+  import Flash from '../../../flash';
   import markdownHeader from './header.vue';
   import markdownToolbar from './toolbar.vue';