'Verify repos script' runs in deprecated govuk-saas-config

[AgaDufrat]

Cause

The Replatforming team have introduced some automation with Terraform in alphagov/govuk-infrastructure#529 which creates Github resources. However there is already a precedent for configuring Github which is applied in govuk-saas-config.

We expect that eventually we'll want remove govuk-saas-config and move its remaining configuration into the Terraform module.

Consequences

1. GitHub configuration is split between two places (though they both link to each other from their READMEs).
2. It might therefore take slightly longer on the rare occasions when someone needs to figure out where some piece of GitHub configuration is coming from.

Solution

We should migrate 'Verify repos script' from govuk-saas-config to Seal

It's defined as a rake task and workflow.