Escape '_' in all parts of a string when escaping for Markdown.

While our Markdown formatting through marked.js technically allows '_'
to be unescaped in the middle of words without problems, it breaks
down in practice. There are bugs in marked.js that can cause '_' in
different parts of a string to be turend into emphasis.

Until that's fixed, or we move to server-side Markdown rendering, we'll
need to disable the escaping of '_' within words in order to prevent
these problems.

Reviewed at https://reviews.reviewboard.org/r/5857/
Use "pre"-mode whitespace rules for <p> tags in Markdown.

Free-form text (<p> tags) are now set to not strip whitespace, causing
them to act just like a <pre> tag. This allows things like indentation
to remain (which Review Bot currently sets).

This is how it used to be, but that was changed in one of the betas to
work around some issues. However, that change was made to *all*
elements, and I don't believe the original issue was with paragraph
text.

Reviewed at https://reviews.reviewboard.org/r/5858/