Provide a concrete class for one-off remarkup blocks

Summary: I want to use some of these for instructional text in Diffusion. Provide a concrete class to make one-offs cacheable and switch Releeph to use it.

Test Plan: {F44175}

Reviewers: btrahan, chad, edward

Reviewed By: btrahan

CC: aran

Differential Revision: https://secure.phabricator.com/D6032