Adds markdown support on ticket details.

Users can now use markdown to author html markup when creating
or editing tickets. Markdown editing is facilitated via the
PageDown markdown javascript editor, and is previewable in a
seperate tab. Rather than using PageDown's js tool for previewing
the content, the system sends the content to the server where it is
converted, and the rendered content is inserted into the preview tab.
This makes for a more consistent result between preview and final markup
(indeed there are no differences now). There were subtle differences between
the results of the PageDown conversion and the backend conversion which
motivated the ajax preview.