Related issues FE -- Step 2: Add AddIssuableForm

See Base MR: https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/1797

• Add AddIssuableForm

---

easy testing `issuable_bundle.js`

import Vue from 'vue';

require('./time_tracking/time_tracking_bundle');

document.addEventListener('DOMContentLoaded', () => {
  const relatedIssuesRootElement = document.querySelector('.js-task-list-container');
  if (relatedIssuesRootElement) {
    // eslint-disable-next-line no-new
    new Vue({
      el: relatedIssuesRootElement,
      components: {
        issueToken: require('./related_issues/components/add_issuable_form.vue'),
      },
      render: createElement => createElement('issue-token', {
        props: {
          inputValue: '',
          addButtonLabel: 'asddfa',
        },
      }),
    });
  }
});