(stable) Wrap edge transaction readers in a translation layer

Summary:
Ref T13051. This puts a translation layer between the raw edge data in the transaction table and the UI that uses it.

The intent is to start writing new, more compact data soon. This class give us a consistent API for interacting with either the new or old data format, so we don't have to migrate everything upfront.

Test Plan: Browsed around, saw existing edge transactions render properly in transactions and feed. Added and removed subscribers and projects, saw good transaction rendering.

Reviewers: amckinley

Reviewed By: amckinley

Maniphest Tasks: T13051

Differential Revision: https://secure.phabricator.com/D18946