Add folder redirects.

Handle redirects around folders correctly. A request for
"www.somesite.com/somefolder" should redirect to
"somesite.com/somefolder/" (removing the www and adding the trailing
slash). A request for "somesite.com/somefolder" shouldn't explicitly
redirect, but the server will auto-redirect to
"somesite.com/somefolder/" (adding the trailing slash). Lastly, a
request for "somesite.com/somefolder/" shouldn't redirect at all.