Previously we changed all CDN links to schemaless.
This is desirable for non HTTPS sites, to ease migration to HTTPS.
It is not desirable for secure sites.
Once site is secure or CDN is secure a rebake should be required
to move it back to non-secure.
- new hidden site setting 'migrate_to_new_scheme' (defaults to false)
- new rake tasks to toggle migration to new scheme
- FIX: migrate_to_new_scheme also works with CDN
- PERF: improve perf of the DbHelper.remap method
- REFACTOR: UrlHelper is now a class
