* Alerting: Rename remote.ExternalAlertmanager to remote.Alertmanager
* Alerting: Send alerts to the remote Alertmanager
* add ticker to readiness check, add tests
* use options when creating a new sender.ExternaAlertmanager
* unexport defaultMaxQueueCapacity
* delete unused defaultConfig field
* add debug log line when sending alerts to the remote alertmanager
* move and refactor readiness check
* update tests to not include defaultConfig
* Alerting: Move `ExternalAlertmanager` to its own package
We'll avoid import cycles when using components from other packages. In addition to that, I've created an `Options` approach for the multiorg alertmanger to allow us to override how per tenant alertmanagers are created.
* switch things around
* address review comments
* fix references and warnings