Clear wagtail_site_root_paths on Site delete

Fixes #706
Merge branch 'clear-site-root-paths-on-delete' of https://github.com/kaedroho/wagtail into kaedroho-clear-site-root-paths-on-delete
fix inaccurate comments and duplicate definition of test_cache_clears_when_site_deleted
release note for #719
Merge branch 'kaedroho-clear-site-root-paths-on-delete'