Fix signin with OmniAuth providers

OmniAuth CSRF protection was broken with the move to Rails 4.2 since the CSRF logic in Rails changed.

This new implementation calls out to Rails instead of copying its code, which is far easier to maintain.

config/initializers/omniauth.rb

@@ -16,7 +16,7 @@ OmniAuth.config.allowed_request_methods = [:post]
#In case of auto sign-in, the GET method is used (users don't get to click on a button)
OmniAuth.config.allowed_request_methods << :get if Gitlab.config.omniauth.auto_sign_in_with_provider.present?
OmniAuth.config.before_request_phase do |env|
-  OmniAuth::RequestForgeryProtection.new(env).call
+  OmniAuth::RequestForgeryProtection.call(env)
end
if Gitlab.config.omniauth.enabled