Include private contributions in user contribution graph

What does this MR do?

This adds an option in user settings to opt-in for including private contributions in user contribution graph.

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

I tried to replicate previous work done in !8495 (closed), taking into account some coding remarks in the discussion.

Screenshots (if relevant)

User settings	Calendar activity (public)	Calendar activity (private)	Calendar, Contributions, Recent activity

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Tests added for this feature/bug
• Review
  • Has been reviewed by UX
  • Has been reviewed by Frontend
  • Has been reviewed by Backend

What are the relevant issue numbers?

Closes #14078 (closed)