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
- 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-qamanual pipeline job)