Redirect /firefox/mobile/ to /firefox/new/

[Bug 1071318](https://bugzilla.mozilla.org/show_bug.cgi?id=1071318)

Moving this redirect from the PHP .htaccess file (see https://bugzilla.mozilla.org/show_bug.cgi?id=1071318#c3) into bedrock so we can more easily/quickly update it when the new /firefox/android page is ready.