Update Rails to version 5.1.1 (#3121)

* Update rails to version 5.1.1

* Run `rails app:update`

* Remove the override of polymorphic activity relationship

* Silence warning about otp_secret attribute being unknown to rails

* We will only introduce form_with where we want to use remote data

app/models/user.rb

@@ -58,6 +58,11 @@ class User < ApplicationRecord
 
   before_validation :sanitize_languages
 
+  # This avoids a deprecation warning from Rails 5.1
+  # It seems possible that a future release of devise-two-factor will
+  # handle this itself, and this can be removed from our User class.
+  attribute :otp_secret
+
   def confirmed?
     confirmed_at.present?
   end