Initialize Rack::Sendfile with accel mappings

Instead of requiring the X-Accel-Mapping header to be sent by nginx,
this allows separation of concerns between application and server
configuration.

Example:
config/initializers/rack_sendfile.rb:
```ruby
Example::Application.configure do
  config.middleware.swap Rack::Sendfile, Rack::Sendfile,
    config.action_dispatch.x_sendfile_header,
    [[Rails.root.join('private', 'uploads'), '/private_uploads']]
end
```