Add invitation system (#398)

spec/mailers/previews/invitation_mailer_preview.rb

@@ -0,0 +1,13 @@
+# Preview all emails at http://localhost:3000/rails/mailers/invitation_mailer
+
+class InvitationMailerPreview < ActionMailer::Preview
+  def initialize(params={})
+    super(params)
+
+    Current.tenant = Tenant.first
+  end
+
+  def invite
+    InvitationMailer.invite(to: 'test@example.com', subject: 'Invitation', body: 'Invitation body')
+  end
+end