PERF: allow looking up incoming email by user_id

This speeds up the ability to remove users from the system

db/migrate/20190426123026_add_incoming_email_by_user_id_index.rb

@@ -0,0 +1,5 @@
+class AddIncomingEmailByUserIdIndex < ActiveRecord::Migration[5.2]
+  def change
+    add_index :incoming_emails, [:user_id], where: 'user_id IS NOT NULL'
+  end
+end