Fix Epic sidebar datepicker

What does this MR do?

Fixes a ~bug where datepicker in Epic sidebar won't open when clicking on Edit button.

Does this MR meet the acceptance criteria?

• Tests added for this feature/bug
• Tested in all supported browsers
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides (https://about.gitlab.com/handbook/engineering/quality/guidelines/test-engineering/).
• EE specific content should be in the top level /ee folder

Closes #10777 (closed)