Show week number in calenders

What does this MR do?

Patch shows the week number in "due date" and other calenders.

Why was this MR needed?

When communicating about timeframes or deadlines, it is common to speak of week numbers. E.g. "we will start in calendar week 23" or "must be ready in week 42".

It would be nice to see this number in the calendar which is used for "due dates". The "Pikaday" calender supports to display this information already by setting the 'showWeekNumber" attribute.

Screenshots (if relevant)

NOTE: in the screenshot, weeks start at monday. This is caused by another patch; the actual one adds only the week numbers

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
• End-to-end tests pass (package-and-qa manual pipeline job)