Make captcha activable for register and comment actions from the XWiki adminstration UI

Currently there is no way for a wiki admin to activate the captcha on register and comment actions.

Provide a way for doing this. It means :

• adapting registerinline.vm to display a captcha image and a response field if the wiki preference for the register action of annonymous users is set to "true"
• same for the comment UI + probably adapt the Java CommentAction to do the check (the register action does this already)
• displaying the fields in the administration for the user to change preferences. In section "registration" an maybe "general" for the comments, or in a new section for comments, to be discussed.