Add iD editor

iD sources live in vendor/assets/iD, and are managed with
vendorer, using the "rewrite" functionality to replace relative
paths in the CSS with <%= asset_path(…) %> directives.

To avoid needing to namespace all of its CSS classes, iD
is embedded via an iframe.

To setup, edit application.yml to include an `id_key` key.
(It can use the same consumer key as P2 if you like.)

config/routes.rb

@@ -131,6 +131,7 @@ OpenStreetMap::Application.routes.draw do
   match '/logout' => 'user#logout', :via => [:get, :post]
   match '/offline' => 'site#offline', :via => :get
   match '/key' => 'site#key', :via => :get
+  match '/id_iframe' => 'site#id_iframe', :via => :get
   match '/user/new' => 'user#new', :via => :get
   match '/user/terms' => 'user#terms', :via => [:get, :post]
   match '/user/save' => 'user#save', :via => :post