Preserve linebreaks again in Summary / Test Plan for revisions

Summary:
I pulled these into the property list recently, which made them more consistent, but that dropped "preserve linebreaks". Since these usually come from the CLI, render with linebreaks preserved.

@csilvers, you'll need to bin/cache purge --purge-remarkup after this if you want to fix existing revisions.

Test Plan: Made a revision with some poetry, saw poetry preserved.

Reviewers: btrahan, csilvers

Reviewed By: btrahan

CC: csilvers, aran

Differential Revision: https://secure.phabricator.com/D7464