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 to dictionaries. Send those to the job queue, rebuild them into objects again, and we end up in the same spot with a little less code duplication.

Finally, delete the mostly-copied code.

Test Plan: Used bulk editor to add comments, projects, and rename tasks.

Reviewers: amckinley

Reviewed By: amckinley

Maniphest Tasks: T13025

Differential Revision: https://secure.phabricator.com/D18875