Allow to order epics by start or end date

What does this MR do?

This implements sorting used on epics roadmap page as described in https://gitlab.com/gitlab-org/gitlab-ee/issues/3559#sorting-logic-of-epics

It sorts epics by start_date, if an epic does not have start_date set, it uses end_date in sorting.

This sorting is not used anywhere yet (so changelog entry is not needed), it will be used on epics roadmap page (#3559 (closed)).

Does this MR meet the acceptance criteria?

• 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)