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/example.application.yml

@@ -78,6 +78,8 @@ defaults: &defaults
   #potlatch2_key: ""
   # OAuth consumer key for the web site
   #oauth_key: ""
+  # OAuth consumer key for iD
+  #id_key: ""
   # Whether to require users to view the CTs before continuing to edit...
   require_terms_seen: false
   # Whether to require users to agree to the CTs before editing