Do not send organiser reminders with "send X days after the camp ends" trigger if WordCamp didn't happen

[timiwahalahti]

We sent organisers reminders with "send X days after the camp ends" trigger even if the WordCamp didn't happen. Especially during the pandemic, many WordCamps were put back to pre-planning but had dates in the tracker. This can also happen from time to time for various reasons.

The wcor_send_after emails were sent out due to the $pending_wordcamps query grabbing those emails, and in timed_email_is_ready_to_send function only checking the end date even the function is called from send_timed_emails for various different WordCamp statuses.

This PR adds a check against the status of WordCamp in timed_email_is_ready_to_send function for wcor_send_after trigger. Only public WordCamp statuses (in schedule, closed) do get the email.

Also, some minor code refactoring was done to make conditions easier to read and tests were updated for this scenario.

Fixes #496

Props @coreymckrill

How to test the changes in this Pull Request:

1. Schedule some WordCamps with pre-planning statuses and past date(s)
2. Run wcor_send_timed_emails cron job

[renintw]

Tested and Looks good. Thanks for some refactorings here!