is commenting.
(add_comment): Check that the adder can read the text he is adding
	a comment link to.
(sub_comment): The user does not have to be able to read either of
	the texts to remove the comment link.  On the other hand, it
	doesn't help if he is supervisor of the author of the parent.
(sub_footnote): The user does not have to be able to read either
	of the texts to remove the footnote link.  He must be supervisor
	of the author of the footnote, but not necessarily the author.