Improve behavior of inline rendering with unified views

Summary:
Ref T2009. This reduces how buggy inlines are. They're still buggy.

Specifically, the inline endpoint didn't know how to scaffold inlines before, so some of them ended up rendering in the wrong rows or breaking layouts.

This passes the current renderer through to the inline editor endpoint, so it can at least get the layout correct.

Test Plan: Interacted with inlines in unified and side-by-side views.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T2009

Differential Revision: https://secure.phabricator.com/D11988