Enable fast task lists for merge requests and epics

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 and epics.

CE version: https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/24779

What are the relevant issue numbers?

https://gitlab.com/gitlab-org/gitlab-ce/issues/19745

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](https://docs.gitlab.com/ee/development/README.html#databases-guides)
• ~~[ ] Link to e2e tests MR added if this MR has Requires e2e tests label. See the Test Planning Process.~~
• EE specific content should be in the top level /ee folder
• [ ] For a paid feature, have we considered GitLab.com plans, how it works for groups, and is there a design for promoting it to users who aren't on the correct plan?
• Security reports checked/validated by reviewer