Install devise invitable

Devise invitable gem is required to invite users.

db/migrate/20190603143107_devise_invitable_add_to_users.rb

@@ -0,0 +1,23 @@
+class DeviseInvitableAddToUsers < ActiveRecord::Migration[5.1]
+  def up
+    change_table :users do |t|
+      t.string     :invitation_token
+      t.datetime   :invitation_created_at
+      t.datetime   :invitation_sent_at
+      t.datetime   :invitation_accepted_at
+      t.integer    :invitation_limit
+      t.references :invited_by, polymorphic: true
+      t.integer    :invitations_count, default: 0
+      t.index      :invitations_count
+      t.index      :invitation_token, unique: true # for invitable
+      t.index      :invited_by_id
+    end
+  end
+
+  def down
+    change_table :users do |t|
+      t.remove_references :invited_by, polymorphic: true
+      t.remove :invitations_count, :invitation_limit, :invitation_sent_at, :invitation_accepted_at, :invitation_token, :invitation_created_at
+    end
+  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: 20181113195810) do
+ActiveRecord::Schema.define(version: 2019_06_03_143107) do
 
   create_table "answers", force: :cascade do |t|
     t.string "title"
@@ -587,8 +587,20 @@ ActiveRecord::Schema.define(version: 20181113195810) do
     t.boolean "is_admin", default: false
     t.string "username"
     t.boolean "is_disabled", default: false
+    t.string "invitation_token"
+    t.datetime "invitation_created_at"
+    t.datetime "invitation_sent_at"
+    t.datetime "invitation_accepted_at"
+    t.integer "invitation_limit"
+    t.string "invited_by_type"
+    t.integer "invited_by_id"
+    t.integer "invitations_count", default: 0
     t.index ["confirmation_token"], name: "index_users_on_confirmation_token", unique: true
     t.index ["email"], name: "index_users_on_email", unique: true
+    t.index ["invitation_token"], name: "index_users_on_invitation_token", unique: true
+    t.index ["invitations_count"], name: "index_users_on_invitations_count"
+    t.index ["invited_by_id"], name: "index_users_on_invited_by_id"
+    t.index ["invited_by_type", "invited_by_id"], name: "index_users_on_invited_by_type_and_invited_by_id"
     t.index ["reset_password_token"], name: "index_users_on_reset_password_token", unique: true
     t.index ["username"], name: "index_users_on_username", unique: true
   end