Prevent embedded remarkup content from cycling when it contains embedded self-references

Summary: Ref T13678. When remarkup content embeds other remarkup content, detect and degrade if the references have nesting depth greater than 1. This is a coarse cycle detector, since rendering shallow (but technically non-cycling) trees doesn't seem valuable.

Test Plan: Created various objects with self-references, saw everything degrade properly (after one level of embedding) when embedded in itself and in other contexts. See attached screenshot.

Maniphest Tasks: T13678

Differential Revision: https://secure.phabricator.com/D21810