Log newsletter emails sent

With this commit we are logging which emails have already received the
newsletter

This could be important if something goes wrong sending the newsletter,
to be able to identify which users have already received the newsletter
and be able to skip them

We’ve had to add a new action to the Activity model (email) and add
paranoia features to be able to deal gracefully with the default
`with_hidden` scope in Activities[1]

[1]
https://github.com/AyuntamientoMadrid/consul/blob/master/app/models/acti
vity.rb#L2

db/migrate/20180517141120_add_hidden_at_to_newsletters.rb

@@ -0,0 +1,5 @@
+class AddHiddenAtToNewsletters < ActiveRecord::Migration
+  def change
+    add_column :newsletters, :hidden_at, :datetime
+  end
+end

db/schema.rb

@@ -682,6 +682,7 @@ ActiveRecord::Schema.define(version: 20180711224810) do
     t.date     "sent_at"
     t.datetime "created_at",        null: false
     t.datetime "updated_at",        null: false
+    t.datetime "hidden_at"
   end
 
   create_table "notifications", force: :cascade do |t|