Minor HTML tweak to question page

According to http://stackoverflow.com/q/3558119/ the <i> tag is not
one of the tags that is allowed to use self-closing syntax, which
means that <i class="icon-foo" /> is equivalent to an open <i> tag
with no closing tag, if the HTML 5 rules were strictly followed.
Most browsers will do the right thing, but it's still not quite in
compliance with the spec -- and since at least one of my editing
tools is complaining about the non-compliance, I'm fixing it.
Add cancel link for answer, comment editor

This could also be implemented as a button. I chose to make it a
link because the visual difference is greater that way. That's also
how StackOverflow does it, probably for the same reason.