R1:65ddefad8b9c

Migrate all Differential reviewer data to edges

Summary:
Ref T1279. @champo did a lot of this work already; we've been doing double writes for a long time.

Add "double reads" (reading the edge table as both the "relationship" table and as the "reviewer status" table), and migrate all the data.

I'm not bothering to try to recover old reviewer status (e.g., we could infer from transactions who accepted old revisions) because it wold be very complicated and doesn't seem too valuable.

Test Plan…