Create dynamic frontend fixture for related issuable mock data
<!--IssueSummary start-->
<details>
<summary>
Everyone can contribute. [Help move this issue forward](https://handbook.gitlab.com/handbook/marketing/developer-relations/contributor-success/community-contributors-workflows/#contributor-links) while earning points, leveling up and collecting rewards.
</summary>
- [Close this issue](https://contributors.gitlab.com/manage-issue?action=close&projectId=278964&issueIid=28282)
</details>
<!--IssueSummary end-->
We should generate the mock data in [`spec/javascripts/vue_shared/components/issue/related_issuable_mock_data.js`](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/spec/javascripts/vue_shared/components/issue/related_issuable_mock_data.js) dynamically from the Rails controller.
issue