SPL: Remove leave questions, birth factcheck, content changes 16/10

Following yesterday's decision by John/Nick/Chris/Roo, we are deleting the leave questions from the SPL questioning process. 

This PR is to be reviewed commit by commit. 

1) Removes all leave questions, some content changes with "todo" made to snippets to silence errors caused by missing date_leave variables
2) Go through birth factcheck tables and amend flows to correct any legal requirement errors I could spot 
3) New content that reflects lack of date_leave information for snippets

Still to do (in future PR)

- Adoption factcheck
- Remove plugins that are no longer used