partial WIKInization

I suggest that WIKIified posts be able to have portions of the post be off-limits to wiki-style editing.

You may want to have a preamble that should not be changed and right now the only way to do that is to write a regular post and then say “go to the wiki in the next post”. If you create a thread for the sake of it being a wiki, the easiest way for new folks to find the wiki is for it to be in the first post. Having to make it the second post so your ineditable pre-amble can occupy the first post seems to confuse people and they can’t find the wiki at all.