Reduce memory usage slightly in za_hansard_load_into_sayit

Only store section ids rather than whole section objects for the summary
at the end.
Find section parent in importer class

Find the parent of the section from within the importer class to avoid
saving the section twice with two different slugs, which can cause a
conflict with other sections.
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.
Add --delete-existing flag to za_hansard_check_for_new_sources

Remove existing sources before running the script. This allows
up-to-date sources to be retrieved.