Summon comment box to the top of the page.

#1545922: [META] Issue page redesign follow-up:

Now that we're going to have all files attached in a single place at the top of the page, people might want to comment right after reviewing the latest patch. It would be useful to either have a "Add new comment" link that scrolls to the section of the page where the add comment box is or do this fancy thing that bugzilla does: