Install devise invitable

Devise invitable gem is required to invite users.

db/migrate/20190603143107_devise_invitable_add_to_users.rb

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+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