Alerting: Send alerts to external Alertmanager(s) (#37298)

* 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.

go.sum

@@ -213,6 +213,7 @@ github.com/alecthomas/template v0.0.0-20190718012654-fb15b899a751/go.mod h1:LOuy
 github.com/alecthomas/units v0.0.0-20151022065526-2efee857e7cf/go.mod h1:ybxpYRFXyAe+OPACYpWeL0wqObRcbAqCMya13uyzqw0=
 github.com/alecthomas/units v0.0.0-20190717042225-c3de453c63f4/go.mod h1:ybxpYRFXyAe+OPACYpWeL0wqObRcbAqCMya13uyzqw0=
 github.com/alecthomas/units v0.0.0-20190924025748-f65c72e2690d/go.mod h1:rBZYJk541a8SKzHPHnH3zbiI+7dagKZ0cgpgrD7Fyho=
+github.com/alecthomas/units v0.0.0-20210208195552-ff826a37aa15 h1:AUNCr9CiJuwrRYS3XieqF+Z9B9gNxo/eANAJCF2eiN4=
 github.com/alecthomas/units v0.0.0-20210208195552-ff826a37aa15/go.mod h1:OMCwj8VM1Kc9e19TLln2VL61YJF0x1XFtfdL4JdbSyE=
 github.com/alicebob/gopher-json v0.0.0-20200520072559-a9ecdc9d1d3a/go.mod h1:SGnFV6hVsYE877CKEZ6tDNTjaSXYUk6QqoIK6PrAtcc=
 github.com/alicebob/miniredis v2.5.0+incompatible/go.mod h1:8HZjEj4yU0dwhYHky+DxYx+6BMjkBbe5ONFIF1MXffk=