Instead of hard-coding the scheme and host everywhere, use _path
methods to show relative URLs.
Except that our previous setting of
Rails.application.routes.default_url_options in
config.after_initialize made this moot because Rails inserts that
host into all _path helpers for some reason. So revert that
setting.
But then anything that wants an absolute URL doesn't know the
hostname and the root_url helper throws an exception. So make a
Rails.application.root_url shortcut to pass the per-app settings in
Rails.application to root_url.
Now we can just use _path helpers most places but still use _url
ones where we need them, such as in RSS views and e-mail templates.
