Fix datepicker initialization on invalid date

What does this MR do?

• Adds projects:milestones:create and groups:milestones:create to dispatcher
• Changes the datepicker selector in the js-clear-due-date resp js-clear-start-date click handler in order to match nested datepicker when validation errors are displayed.

Why was this MR needed?

The milestone calendar/datepicker was not working after failed validation of data in "New milestone". Moreover, the 'clear start/due date' buttons did not clear the date inputs. (For a detailed description, please see https://gitlab.com/gitlab-org/gitlab-ce/issues/32066)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• API support added
• Tests added for this feature/bug
• Review
  • Has been reviewed by UX
  • Has been reviewed by Frontend
  • Has been reviewed by Backend
  • Has been reviewed by Database
• Conform by the merge request performance guides
• Conform by the style guides
• Squashed related commits together
• Internationalization required/considered

What are the relevant issue numbers?

Closes #32066 (closed)