R1:7276af6a81f4

Make yellow "draft" bubbles more generic

Summary:
Fixes T12095. Ref T6660. The old code for this was specific to Differential, using the `DifferentialDraft` table.

Instead, make the `EditEngine` / `VersionedDraft` code create and remove a `<objectPHID, authorPHID>` edge when a particular author creates drafts.

Some applications have drafts beyond `VersionedDrafts`, notably inline comments. Before writing "yes, draft" or "no, no draft", ask the object if it has any custom draft stuff we need to know…