Downloadable File Fixes/Tweaks

* Hooked up new downloadable product permissions function to
completed/processing actions
* WC_Product::get_files() is used rather than the underlying
_downloadable_files meta directly
* $this is passed to downloadable product filters in WC_Product in place
of $this->id which doesn't does not afford you the particular variation
when a product variation is being used.
Merge pull request #4386 from justinstern/master

Downloadable File Fixes/Tweaks