a8n: Filter out repositories of unsupported external services

Created by: mrnugget

We currently only support GitHub and Bitbucket Server.

It doesn't make sense and only leads to bugs if we try to run CampaignJobs over repositories whose external service we don't support, because we later can't turn those CampaignJobs into Changesets on the codehost.

And before manually creating a changeset we check that all repositories are supported.