Backend validation: Epic start date can be after epic end date if you click fast enough

• If you click fast enough, you can get an epic start date that is after the epic end date, which fails the constraint designed in https://gitlab.com/gitlab-org/gitlab-ee/issues/3556.
• The scope of this issue is just backend validation. Any frontend changes required will be scoped out in a separate issue.
• If the backend validation fails, the date is not stored.

• This is an edge case, but not an unreasonable one, especially if your connection is slow or this is some connection glitch while using Gitlab.
• We want to maintain the constraint so that when we have the roadmap view, you won't have ill-defined epics.