install mailkick gem

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

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"