Add --delete-existing flag to za_hansard_load_into_sayit

This ensures that the import is done into a clean database, rather than
duplicating all the existing speeches.
Delete existing hansard sources if running with --check-all

If we don't remove the existing sources then --check-all fails because
there will be existing records for the documents that are being created.
Add --delete-existing flag to za_hansard_q_and_a_scraper

When this flag is supplied the import_into_sayit step will delete
existing speeches before doing the import.