Fixes T12058. When the user visits /maniphest, for example, we redirect to /maniphest/.

Since this redirect is very low-level (at the Aphront level, below the Site level) we need to preserve the request Host rather than correct it to PhabricatorEnv::getURI() or similar -- the request may be hiting a different Site like a blog domain.

Currently, we do not preserve the port. Instead, preserve the port if it is not a standard port for the protocol (80 for http, 443 for https).

• Made a request with a missing slash and a normal port in my browser, got redirected normally.
• Made a request with a missing slash and a nonstandard port, got redirected on the same port.

$ curl -H 'Host: local.phacility.com:123' -v http://local.phacility.com/diffusion
*   Trying 127.0.0.1...
* Connected to local.phacility.com (127.0.0.1) port 80 (#0)
> GET /diffusion HTTP/1.1
...
> 
< HTTP/1.1 302 Found
...
< Location: http://local.phacility.com:123/diffusion/
...