Customize contribution calendar week start (Monday/Sunday)

What does this MR do?

Adds an ability to select Monday as a beginning of the week for contribution calendar. It will remain Sunday by default.

Are there points in the code the reviewer needs to double check?

Calendar JS code is still a bit messy so it's worth double-checking.

Why was this MR needed?

According to #19282 (moved) some people were not happy with having Sunday as a first day of the week. This MR doesn't change anything in default behavior, but adds an option to select Monday as a week start.

Screenshots (if relevant)

Does this MR meet the acceptance criteria?

• CHANGELOG entry added
• Documentation created/updated
• API support added
• Tests
  • Added for this feature/bug
  • All builds are passing
• Conform by the merge request performance guides
• Conform by the style guides
• Branch has no merge conflicts with master (if it does - rebase it please)
• Squashed related commits together

What are the relevant issue numbers?

#19282 (moved)