migrate slackline to ES7

the account is used by a cloud function, created from: https://gitlab.com/gitlab-com/gl-infra/slackline/

We tried switching the function to an ES7 account, but the alerts-gprd index in ES7 is empty, so the function was switched back to ES5 (using new credentials).

The purpose of this issue is to:

investigate what is populating the alerts-gprd index
make sure the contents in ES7 matches ES5
switch the cloud function to a new URL

Edited Mar 02, 2020 by Igor