* Alerting: Send alerts to external Alertmanager(s)
Within this PR we're adding support for registering or unregistering
sending to a set of external alertmanagers. A few of the things that are
going are:
- Introduce a new table to hold "admin" (either org or global)
configuration we can change at runtime.
- A new periodic check that polls for this configuration and adjusts the
"senders" accordingly.
- Introduces a new concept of "senders" that are responsible for
shipping the alerts to the external Alertmanager(s). In a nutshell,
this is the Prometheus notifier (the one in charge of sending the alert)
mapped to a multi-tenant map.
There are a few code movements here and there but those are minor, I
tried to keep things intact as much as possible so that we could have an
easier diff.
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