insights: AddInsightViewToDashboard race condition

Created by: coury-clark

The AddInsightViewToDashboard mutation currently does not operate in a transaction and has a race condition between validations and DB modifications.

https://sourcegraph.com/github.com/sourcegraph/sourcegraph/-/blob/enterprise/internal/insights/resolvers/dashboard_resolvers.go?utm_product_name=GoLand&utm_product_version=GoLand&utm_source=JetBrains-v1.2.1#L363:28