Add openid connect support using doorkeeper-openid_connect gem

... as discussed in [Issue 507](https://github.com/openstreetmap/operations/issues/507)
and described by @mmd-osm.

To activate, set the value of `doorkeeper_signing_key` to RSA private key.

Allows using openstreetmap as an identity provider.

Adds `openid` scope to OAuth2 authorizations, required to login to OSM.

Currently, the only claims returned are:
 - "openid" scope: "sub" and "preferred_username"
 - "read_email" scope: "email"

config/routes.rb

@@ -1,11 +1,12 @@
 OpenStreetMap::Application.routes.draw do
-  use_doorkeeper_openid_connect
   use_doorkeeper :scope => "oauth2" do
     controllers :authorizations => "oauth2_authorizations",
                 :applications => "oauth2_applications",
                 :authorized_applications => "oauth2_authorized_applications"
   end
 
+  use_doorkeeper_openid_connect :scope => "oauth2" if Settings.key?(:doorkeeper_signing_key)
+
   # API
   namespace :api do
     get "capabilities" => "capabilities#show" # Deprecated, remove when 0.6 support is removed