Summary: This is supposed to look a lot like the way Remarkup renders a block of code, so you can render some out of context message inside another container. For example in Releeph, it renders a message someone has associated with a Releeph request.
Test Plan:
I've added an abstract uiexample, but the use case in Releeph is more explanatory:
{F33900}
Reviewers: epriestley, chad
Reviewed By: epriestley
CC: aran, Korvin
Maniphest Tasks: T2094
Differential Revision: https://secure.phabricator.com/D5125
