Add global toast module

**Why?**
https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/15015#note_210891978

app/assets/javascripts/vue_shared/plugins/global_toast.js

+import Vue from 'vue';
+import { GlToast } from '@gitlab/ui';
+
+Vue.use(GlToast);
+
+export default function showGlobalToast(...args) {
+  return Vue.toasted.show(...args);
+}

spec/frontend/vue_shared/plugins/global_toast_spec.js

+import toast from '~/vue_shared/plugins/global_toast';
+import Vue from 'vue';
+
+describe('Global toast', () => {
+  let spyFunc;
+
+  beforeEach(() => {
+    spyFunc = jest.spyOn(Vue.toasted, 'show').mockImplementation(() => {});
+  });
+
+  afterEach(() => {
+    spyFunc.mockRestore();
+  });
+
+  it('should pass all args to Vue toasted', () => {
+    const arg1 = 'TestMessage';
+    const arg2 = { className: 'foo' };
+
+    toast(arg1, arg2);
+
+    expect(Vue.toasted.show).toHaveBeenCalledTimes(1);
+    expect(Vue.toasted.show).toHaveBeenCalledWith(arg1, arg2);
+  });
+});