Implement pushing in SyncRemotesService

When a sync fails, it will log a fatal message, which automatically goes to Sentry, which goes to Slack (in addition to the job log and Elasticsearch). This will give us more visibility into diverged stable branches.

Closes https://gitlab.com/gitlab-org/release-tools/issues/309, https://gitlab.com/gitlab-org/release-tools/issues/308, https://gitlab.com/gitlab-org/release-tools/issues/307