Respect linebreaks in full HTML tables in Remarkup

Summary:
Fixes T5427. See PHI1630. See also T13160 and D20568.

In the full HTML table syntax with "<table>", respect linebreaks as literals inside "<td>" cells.

Test Plan: Previewed some full-HTML tables with and without linebreaks, saw what seemed like sensible rendering behavior.

Maniphest Tasks: T5427

Differential Revision: https://secure.phabricator.com/D20971