Skip CP change emails for users who have withdrawn all forecasts

[palcu]

Summary

• When sending "Significant change" (CP change) emails, skip users who have withdrawn their predictions from ALL questions in a post
• If a user still has at least one active forecast on any question in the post, they will continue receiving CP change emails

Changes

• Add helper function get_users_with_active_forecasts_for_questions() that returns user IDs with at least one active forecast (where end_time is NULL or in the future)
• Modify notify_post_cp_change() to skip users without active forecasts
• Add tests for the new functionality

Test plan

• Added unit tests for get_users_with_active_forecasts_for_questions()
  • Users with active forecasts (end_time=None) are included
  • Users with withdrawn forecasts (end_time in past) are excluded
  • Users with future end_time are still considered active
  • Users with at least one active forecast across multiple questions are included

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