Use "whitespace: pre-wrap" for <tt> text in Remarkup

Summary:
I typed (tilde) (space) (space) (space) (space) (space) (tilde) earlier but it just vanishes, make it do what I intended.

Also fix a missing space that was breaking a CSS rule (in h1), set it to the intended value, and remove redundant margins (h1..h6).

Test Plan: Put some spaces in tildes, made some headers.

Reviewers: vrana, btrahan

Reviewed By: vrana

CC: aran, epriestley

Differential Revision: https://secure.phabricator.com/D1803