Security fixes to how discussions are posted and edited

* Closes CSRF vector in /post/discussion & /post/editdiscussion.
* Disallows changing the Format of a discussion when it shouldn't be.
* Disallow blank Format values; it is going to continue to be a security target otherwise.
If we're removing the special case for blank FormatMethod above, no change is needed here anyway, except maybe to unset Format if ForceInputFormatter is set & we're editing.