Controllers now attempt to send the user to the previous page on error.

As a user-friendly feature, our controllers should do their best to send
the user back to the page they were at before an error occurred instead
of dumping them back on the site's root page every time. Because
redirecting the user back requires the HTTP_REFERER header, we still
send the user back to the root page if they don't have that header set.
Remove a FIXME comment.

The old FIXME requests the user be redirected to their dashboard on
login. In hindsight, it's better to keep the existing behavior of
sending them to the site's root page because that's intended to the main
page for the site.