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

Description

• 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 — Demo GIF

• The scope of this issue is the frontend changes resulting from the backend validation work done in #4002 (closed)

Proposal

• After selecting a date outside of the valid date range (start after finish, or finish before start), the date picker should close and a spinner appear (during this time, the validation occurs)
• If there's an error, show an error message (see image below) and re-open the date picker so the user can select a different date
  • The user can close the date picker by clicking outside of it or clicking the dropdown select. In this case, the error message disappears.