R1:742d45b625d1

Modernize file embed Remarkup rule

Summary: Ref T603. Make this rule properly policy-aware, and extend from `PhabricatorRemarkupRuleObject`.

Test Plan:
- Embedded an image, tested all options (name, link, float, layout, size).
- Used lightbox to view several images.
- Embedded a text file, tested all options (name).
- Embedded audio, tested all options (loop, autoplay).
- Attached a file via comment to a task, verified edge was created.
- Attached a file via comment to a conpherence, verified…