Update epics index JSON response

What does this MR do?

Update epics index JSON response

For JSON requests epics index page returns a JSON array of epics. This will be used on epic roadmap page (#3559 (closed)). Pagination is disabled in this case - it's not supported for the first iteration on roadmap page.

Until now JSON response for epics index action returned an HTML list of epics but it was not used anywhere in our code.

Changelog entry is not needed as this JSON response was not used anywhere yet.

Does this MR meet the acceptance criteria?

• [ ] Changelog entry added, if necessary
• [ ] Documentation created/updated
• [ ] API support added
• Tests added for this feature/bug
• Review
  • Has been reviewed by Backend
• Conform by the merge request performance guides
• Conform by the style guides
• Squashed related commits together
• Internationalization required/considered
• If paid feature, have we considered GitLab.com plan and how it works for groups and is there a design for promoting it to users who aren't on the correct plan
• End-to-end tests pass (package-qa manual pipeline job)

What are the relevant issue numbers?

#3559 (closed)