Migrate Auth provider

Currently using netlify identity looking to migrate to Auth0

Netlify identity doesn't give you access to the JWT secret without being on a (expensive $99/m) paid plan. Plus there's not an easy way to do staging, and broadly speaking it was an easy option when the project was starting.

- [ ] Get sign ups and new login's working with Auth0
- [ ] ensure RBAC works
- [ ] Make sure password reset flow still works
- [ ] Add migration path for existing netlify-identity-users signing in, if they are rejected by auth0, check to see if they are in netlify-identity, if so add them to Auth0
- [ ] Document how to set up Auth0 in contributing.md