Expose the predicted number of a new comment

This was originally part of the `issue.comment.form` plugin, which was removed in 6dbdcc3 (after the d.o major upgrade to D7).

It not only saves you from doing the math manually, it also avoids needless scrolling (since the heading is more likely to be in the visible viewport).

Testing revealed that the current implementation of `issue.getNewCommentNumber()` is completely bogus.