Add warning when there is pending migration

What does this MR do and why?

This will add a warning on running the rake task gitlab:app:check if there is any pending elasticsearch migration. All migrations must be finished before doing a major upgrade. This relates to #376376 (closed)

Screenshots or screen recordings

Pending migrations exist

Pending migrations do not exist

How to set up and validate locally

Create a new ES migration and run gitlab:app:check. You will see the warning message. Complete all the pending migrations and run gitlab:app:check. You will see a green yes message

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.