Decide a proper policy for dev access to the prod Auth0 tenant

[alexwlchan]

This is a question that's come up a couple of times, and I think it would be sensible for us to have a proper policy that's written down somewhere, so we can apply permissions consistently and in an agreed-upon way.

There's some initial discussion in a Slack thread here: https://wellcome.slack.com/archives/C3TQSF63C/p1661415312871959