Preserve trailing whitespace in Remarkup

Summary:
See T522. We'll sometimes incorrectly parse diffs as "code block" + "bulleted
list". There are probably a couple of fixes for this like letting code blocks
have strong forward binding power, but I think this is the easiest and
most-expected approach: stop removing trailing whitespace.

This may cause some confusion in cases where a user enters:

blah graf 1 blah
 << space here
blah graf 2 blah

...and expects to get two paragraphs but only gets one -- but I think that's
pretty rare, and the diff problem has shown up in the wild a few times.

If this causes problems in the wild we can try something more creative.

Test Plan: Unit tests fail before this patch and pass after it.

Reviewers: btrahan, fugalh, jungejason, nh, tuomaspelkonen, aran

Reviewed By: fugalh

CC: aran, fugalh, epriestley, jacobchin

Differential Revision: 1125