Implemented When converting thread, preserve first post

[Bob]

As per title, when converting a thread to an item, some people prefer to preserve the first post of the discussion thread instead of replacing the first post with a new message. Since most people I know prefer to preserve, the option is checked by default (meaning that if you want that canned message to replace the first post of the thread, you need to uncheck the option.

This option is on both single convert and bulk convert forms.