Fixing #230; add a scrollbar to the PM popup dialog if necessary. This wasn't as easy as I thought. Had to wrap the PM text in a div with its styling (max-height and overflow). If I then resized the dialog, I'd get two scrollbars. So for now, made the dialog non-resizable.

Dear {{ to_name }},

Your friend, {{ sender_name }}, wanted to send you a link to an 
interesting story found on {{ site_name }}.

{{ story_title|safe }}
http://{{ site_url }}{{ story_link }}

Thanks!
http://{{ site_url}}