This commit changes some (imo) inconsistent behaviour in Discuss and includes some fixes for previous commits.

When editing a post, the subscribe checkbox will now be displayed only to the original author of the post. Also the checkboxes for stick and lock will only be displayed when a user edits an inital post or creates a new thread and has the rights to do so.

- Enable sticking/unsticking a thread while editing it with the replyform

- Enable locking/unlocking a thread while editing it with the replyform

- Allows subscribing/unsubscribing to a thread while editing it with the replyform for the original author

- The subscribe checkbox now will be rendered using the checkbox chunk.

- Newly created threads are now disThreadDiscussions by default (when not changed in the form)

- Edited threads will now be either disThreadDiscussion or disThreadQuestion (default: disThreadDiscussion)