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.)
Update iD; clean up OAuth configuration
Fix edit links on browse and changeset pages
Call updatelinks when map moves

Fixes systemed/iD#1330.
Fix search results display (systemed/iD#1328)
Load search result locations in iD
Update iD
Support maximize/minimize (systemed/iD#1327)
Fix search result link behavior (systemed/iD#1329)
Selecting relations is now supported
Update iD