monitoring: granular alerts notifications with Alertmanager

Created by: bobheadxi

Right now (as of https://github.com/sourcegraph/sourcegraph/pull/11483), we create a copy of the home dashboard (since the home dashboard is non-editable) to attach alerts to, and alerts basically only tell you if warning alerts are firing or if critical alerts are firing.

We want a expand on https://github.com/sourcegraph/sourcegraph/issues/10641 (where notifications only tell you "warning alerts are firing" and "critical alerts are firing") to get better granularity for alerts (ie provide what specific alert is firing), and be able to support per-alert silencing down the line (#11210 (closed))

Discussions

https://github.com/sourcegraph/sourcegraph/issues/11452#issuecomment-648567565

https://github.com/sourcegraph/sourcegraph/pull/11427/files#r439128693

Updated plan

https://github.com/sourcegraph/sourcegraph/issues/11452#issuecomment-648652514