Add invitation system (#398)

app/mailers/invitation_mailer.rb

@@ -0,0 +1,10 @@
+class InvitationMailer < ApplicationMailer
+  def invite(to:, subject:, body:)
+    @body = body
+
+    mail(
+      to: to,
+      subject: subject
+    )
+  end
+end