Fix regexp matcher for special assets for the browser to clash with
less of the object namespace.
Assets should now be loaded with the /minio/ prefix. Previously,
favicon.ico (and others) could be loaded at any path matching
/minio/*/favicon.ico. This clashes with a large part of the object
namespace. With this change, /minio/favicon.ico will serve the favicon
but not /minio/mybucket/favicon.ico
Fixes#7077
