R1:01253d533bbc

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…