Implemented When converting thread, preserve first post

As per title, when converting a thread to an article, some people prefer to preserve the first post of the discussion thread instead of replacing the first post with a new message. AMS had an option to do this, but it was either ON or OFF for everyone. I've removed that in favor of adding an option to the for allowing the individual that is doing the convert to make the decision instead of always being forced one way or the other. Since most people I know prefer to preserve, the option is checked by default (meaning that if you want that stupid 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.