R1:1e51f1d0dc25

Reuse more transaction construction code in bulk editor

Summary:
Ref T13025. Currently, the bulk editor takes an HTTP request and emits a list of "raw" transactions (simple dictionaries). This goes into the job queue, and the background job builds a real transaction.

However, the logic to turn an HTTP request into a raw transaction is ending up with some duplication, since we generally already have logic to turn an HTTP request into a full object.

Instead: build real objects first, then serialize them…