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 edge was created.
• Viewed old files, verified remarkup version bump rendered them correctly.

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T603

Differential Revision: https://secure.phabricator.com/D7192