frontend: disallow non-admins from using TriggerObservabilityTestAlert
Created by: bobheadxi
I suspect that somebody set off https://sourcegraph.app.opsgenie.com/alert/detail/7921f57e-fae0-4f21-99bc-fa3060409339-1597066049210/details , since I just realized I did not set a guard on this endpoint (https://github.com/sourcegraph/sourcegraph/pull/12532)
Looking into logs now to see if I can confirm if the last person to use this endpoint is in our org