Fix #66 Comment submission reset problem

When submitting a comment from the comments page the type of the
comment should be FORUM. However it was only that for the first
submission, and subsequent POSTs had an empty type. This causes
problems with the comment retrieval because when we pull comments
we retrieve only those with the types specified in bb1c1f88

This resolves the problem by modifying the clearDialogSliderInputs
function to set the default value of the comment_type hidden field
to FORUM.