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