R1:64cee4a902da

Move Phriction internal document/content references from IDs to PHIDs

Summary:
Ref T13077. This is mostly just a small cleanup change, even though the actual change is large.

We currently reference content and document objects from one another with `contentID` and `documentID`, but this means that `contentID` must be nullable. Switching to PHIDs allows the column to be non-nullable.

This also supports reorienting some current and future transactions around PHIDs, which is preferable for the API. In…