Skip admin authentication for user oauth apps

Upgrading doorkeeper to 5.0.2 introduced a change where the method
`admin_authenticator` is required to be defined in the config, otherwise
all responses from the Doorkeeper will be a 403.

https://github.com/doorkeeper-gem/doorkeeper/wiki/Migration-from-old-versions#api-changes-3

How the admin authenticator behaves for our applications depends on the
context that the application is being created in - so a base level
definition of this will be complicated.

For now, skip this new before action in the controller. We already have
user level authentication in this context.

app/controllers/oauth/applications_controller.rb

@@ -8,6 +8,8 @@ class Oauth::ApplicationsController < Doorkeeper::ApplicationsController
   include Gitlab::Experimentation::ControllerConcern
   include InitializesCurrentUserMode
 
+  skip_before_action :authenticate_admin!
+
   prepend_before_action :verify_user_oauth_applications_enabled, except: :index
   prepend_before_action :authenticate_user!
   before_action :add_gon_variables