Refactored activity calendar

What does this MR do?

From the original contributor

Previous implementation made it difficult to change the first-day-of-week because '0' (sunday) was assumed at several places.

Patch adds an attribute 'firstDayOfWeek' and bases e.g. calculation of y coordinates on it.

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Tests added for this feature/bug
• Review
  • Has been reviewed by Frontend
• Conform by the merge request performance guides
• Conform by the style guides
• Squashed related commits together

What are the relevant issue numbers?

No issue number, but this is the original MR: https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/17980