Construct redirect route set from content item.

Add support for constructing the corresponding route set for a redirect
format content item.
Set validation errors on the corresponding fields.

All route set validation errors were being set on the routes field on
ContentItem.  Now that there are 2 fields relating to these validations,
the errors need to be set on the appropriate one.
Remove rendering_app field from RegisterableRoute

It was not being used by anything.  The rendering_app is set on the
RegisterableRouteSet class as well, and that's the value that actually
gets used when registering routes.
Register redirect routes with router.