Add feature flag toggle for elastic migration worker

What does this MR do and why?

From the handbook section on feature flags:

Don’t use feature flags at application load time. For example, using the Feature class in config/initializers/* or at the class level could cause an unexpected error

This feature flag, when disabled, makes Advanced Search migrations a no-op. This is useful because we usually want to monitor a migration when it starts in case it goes wrong. Moving forward, we will most likely keep it disabled until we are ready for migrations to be picked up the cron worker and executed.

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