monitoring: migrate existing alert rules to generator

Created by: bobheadxi

From https://github.com/sourcegraph/sourcegraph/issues/5370#issuecomment-629406540 by @slimsag:

Make sure our existing Alertmanager rules are fully encapsulated already by what the monitoring generator covers alerts-wise. This may require introducing a Sourcegraph.com-specific mode to the monitoring generator, but I'd like to avoid that if at all possible.