Insights: make it so any user, not just site admins, can create global dashboards

Created by: Joelkw

We want any user to be able to create a global dashboard, not just admins. (See https://sourcegraph.slack.com/archives/C024QE1K0FP/p1643159820013800?thread_ts=1643139478.012200&cid=C024QE1K0FP). We should remove checks that prevent non-admins from creating a global dashboard – those checks originally were created here, it seems.