[Bug 906863] Preserve newlines in reviews.

Thanks to @willkg for suggesting nl2br, which made this a lot easier.

To test this, I opened a private browser window, logged into a second account, made an KB edit, then went back to my normal session and approved it. This made my ./manage.py terminal spit out an email and that had html in it that I could look at and/or shove into a file and render in Firefox.

r?