Enable fast task lists for merge requests

What does this MR do?

Enables the new fast task list code (https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/23938) for merge requests.

What are the relevant issue numbers?

Closes #19745 (closed)

Does this MR meet the acceptance criteria?

• [ ] Changelog entry added, if necessary
• [ ] Documentation created/updated via this MR
• [ ] Documentation reviewed by technical writer or follow-up review issue created
• Tests added for this feature/bug
• Tested in all supported browsers
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• [ ] Conforms to the database guides
• [ ] Link to e2e tests MR added if this MR has Requires e2e tests label. See the [Test Planning Process](https://about.gitlab.com/handbook/engineering/quality/test-engineering/).
• Security reports checked/validated by reviewer