Add index services on template

What does this MR do?

This migration is a corrective action to add the missing index_services_on_template index on staging.

We suspect the column rename was executed in staging already before it was rolled back/reverted with 8d4d9705. The rollback doesn't know about the existence of the index though and hence it is now missing.

Context https://gitlab.slack.com/archives/C8PKBH3M5/p1583649161054100

$ rails db:migrate
== 20200309105539 AddIndexServicesOnTemplate: migrating =======================
-- index_exists?(:services, :template)
   -> 0.0040s
== 20200309105539 AddIndexServicesOnTemplate: migrated (0.0041s) ==============