monitoring: disable critical alerts banner by default
Created by: bobheadxi
This banner is currently a significant source of confusion, and since we're still working on dogfooding alerts in sourcegraph.com (https://github.com/sourcegraph/sourcegraph/issues/5370) it's hard to say we're confident about all our alerts enough to have it displayed so prominently (yet)
This changes the default of alerts.hideObservabilitySiteAlerts
to true
Most recent Slack discussion: https://sourcegraph.slack.com/archives/C07KZF47K/p1594730258245600