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.

{% load url from future %}
{% if show_button %}
<a href="{% url 'forums-edit_post' id=post.id %}"><img src="{{ STATIC_URL }}icons/page_edit.png" alt="Edit post" title="Edit post" /></a>
{% endif %}