Remove form_action restrictions for sessions#login

Login may redirect to ouath2_authorizations#create which may then
redirect to arbitrary schemes if the application is already authorized
so we need to allow login to redirect to any scheme.

Fixes #3424

app/controllers/sessions_controller.rb

@@ -12,9 +12,7 @@ class SessionsController < ApplicationController
   authorize_resource :class => false
 
   def new
-    append_content_security_policy_directives(
-      :form_action => %w[*]
-    )
+    override_content_security_policy_directives(:form_action => []) if Settings.csp_enforce || Settings.key?(:csp_report_url)
 
     session[:referer] = safe_referer(params[:referer]) if params[:referer]
   end