Persist remarkup metadata in "VersionedDrafts" and record explicit file uploads

Summary:
Ref T13603. Allow "VersionedDraft" to persist remarkup comment area metadata from stacked actions controls.

When files are dragged and dropped, record them as explicit uploads in comment metadata.

Test Plan: Dragged and dropped files into Remarkup stacked action text areas (e.g., in Maniphest), reloaded page, saw metadata persist across reloads.

Maniphest Tasks: T13603

Differential Revision: https://secure.phabricator.com/D21828