install mailkick gem

this gem is used to allow users to unsubscribe from the invitation email

Gemfile

@@ -223,6 +223,9 @@ gem 'nokogiri', '>= 1.8.1'
 # memcached binary connector
 gem 'dalli'
 
+# Unsubscribe from email
+gem 'mailkick'
+
 # Use guard and spring for testing in development
 group :development do
   # to launch specs when files are modified

Gemfile.lock

@@ -280,6 +280,8 @@ GEM
     lumberjack (1.0.12)
     mail (2.7.1)
       mini_mime (>= 0.1.1)
+    mailkick (0.4.0)
+      activesupport (>= 5)
     marcel (0.3.3)
       mimemagic (~> 0.3.2)
     method_source (0.8.2)
@@ -669,6 +671,7 @@ DEPENDENCIES
   json-schema
   leaflet-rails
   letter_opener
+  mailkick
   mina
   mini_magick
   money-rails

app/models/user.rb

@@ -8,6 +8,7 @@ end
 
 class User < ApplicationRecord
   rolify
+  mailkick_user
   # prevent N+1 queries with has_cached_role? by preloading roles *always*
   default_scope { preload(:roles) }
 

db/migrate/20190721210158_install_mailkick.rb

@@ -0,0 +1,14 @@
+class InstallMailkick < ActiveRecord::Migration[5.2]
+  def change
+    create_table :mailkick_opt_outs do |t|
+      t.string :email
+      t.references :user, polymorphic: true
+      t.boolean :active, null: false, default: true
+      t.string :reason
+      t.string :list
+      t.timestamps
+    end
+
+    add_index :mailkick_opt_outs, :email
+  end
+end

db/schema.rb

@@ -10,7 +10,7 @@
 #
 # It's strongly recommended that you check this file into your version control system.
 
-ActiveRecord::Schema.define(version: 2019_06_12_161235) do
+ActiveRecord::Schema.define(version: 2019_07_21_210158) do
 
   create_table "answers", force: :cascade do |t|
     t.string "title"
@@ -278,6 +278,19 @@ ActiveRecord::Schema.define(version: 2019_06_12_161235) do
     t.string "picture"
   end
 
+  create_table "mailkick_opt_outs", force: :cascade do |t|
+    t.string "email"
+    t.string "user_type"
+    t.integer "user_id"
+    t.boolean "active", default: true, null: false
+    t.string "reason"
+    t.string "list"
+    t.datetime "created_at", null: false
+    t.datetime "updated_at", null: false
+    t.index ["email"], name: "index_mailkick_opt_outs_on_email"
+    t.index ["user_type", "user_id"], name: "index_mailkick_opt_outs_on_user_type_and_user_id"
+  end
+
   create_table "openids", force: :cascade do |t|
     t.string "provider"
     t.string "email"