allow extensions that use 'UserCreateForm' to add checkboxes into the signup form...

allow extensions that use 'UserCreateForm' to add checkboxes into the signup form to define messages containing HTML. this can be very useful and necessary; think of something like a checkbox "I have read the site's terms of use", it would obviously need to link to the terms of use page. using msgWiki() instead of msgHtml() here would probably be safer, but msgWiki() likes to mess things up by introducing <p> tags and this is meant for extensions, so it won't cause any more risks than the current existing system messages that allow HTML inclusion.