Feature/update confirmation email information (#145)

* Add `invited_by` foreign key to User

Allows for a User to be tied to the user who invited them

* Include `current_user` in new agent initialization parameters

* Add `shoulda-matchers` for testing associations

* Add Inviter information and associated account to welcome email

* Only show inviter info if applicable

* Update conversation spec for FFaker compatibility

spec/models/user_spec.rb

@@ -0,0 +1,24 @@
+# frozen_string_literal: true
+
+require 'rails_helper'
+
+RSpec.describe User do
+  context 'validations' do
+    it { is_expected.to validate_presence_of(:email) }
+    it { is_expected.to validate_presence_of(:name) }
+    it { is_expected.to validate_presence_of(:account_id) }
+  end
+
+  context 'associations' do
+    it { is_expected.to belong_to(:account) }
+    it { is_expected.to belong_to(:inviter).class_name('User').required(false) }
+
+    it do
+      is_expected.to have_many(:assigned_conversations)
+        .class_name('Conversation').dependent(:nullify)
+    end
+    it { is_expected.to have_many(:inbox_members).dependent(:destroy) }
+    it { is_expected.to have_many(:assigned_inboxes).through(:inbox_members) }
+    it { is_expected.to have_many(:messages) }
+  end
+end