Define custom base controller for Doorkeeper

Since we only need the `can?` view helpers there, it's better to
include those in a separate controller.

If we inherit from `ApplicationController` we also need to deal with
authentication, that needs to be done in some, but not all doorkeeper controllers.

changelogs/unreleased/bvl-fix-openid-redirect.yml

+---
+title: Fix redirection error for applications using OpenID
+merge_request: 18599
+author:
+type: fixed

config/initializers/doorkeeper.rb

@@ -104,5 +104,5 @@ Doorkeeper.configure do
   # set to true if you want this to be allowed
   # wildcard_redirect_uri false
 
-  base_controller 'ApplicationController'
+  base_controller '::Gitlab::BaseDoorkeeperController'
 end

lib/gitlab/base_doorkeeper_controller.rb

+# This is a base controller for doorkeeper.
+# It adds the `can?` helper used in the views.
+module Gitlab
+  class BaseDoorkeeperController < ActionController::Base
+    include Gitlab::Allowable
+    helper_method :can?
+  end
+end

spec/requests/openid_connect_spec.rb

@@ -153,4 +153,13 @@ describe 'OpenID Connect requests' do
       end
     end
   end
+
+  context 'OpenID configuration information' do
+    it 'correctly returns the configuration' do
+      get '/.well-known/openid-configuration'
+
+      expect(response).to have_gitlab_http_status(200)
+      expect(json_response).to have_key('issuer')
+    end
+  end
 end