Add notification mailer, make wallet notifications configurable

app/mailers/notification_mailer.rb

@@ -0,0 +1,8 @@
+class NotificationMailer < ApplicationMailer
+  def lightning_sats_received
+    @user = params[:user]
+    @amount_sats = params[:amount_sats]
+    @subject = "Sats received"
+    mail to: @user.email, subject: @subject
+  end
+end