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"

app/views/oauth2_applications/_form.html.erb

@@ -3,5 +3,5 @@
 <%= f.form_group :confidential do %>
   <%= f.check_box :confidential %>
 <% end %>
-<%= f.collection_check_boxes :scopes, Oauth.scopes(:privileged => current_user.administrator?), :name, :description %>
+<%= f.collection_check_boxes :scopes, Oauth.scopes(:oauth2 => true, :privileged => current_user.administrator?), :name, :description %>
 <%= f.primary %>