Add preconnect and dns-prefetch to nau-richie

[igobranco]

The existing richie preconnect and dns-prefetch on richie base.html isn't working on NAU Richie site.

https://github.com/openfun/richie/blob/0e67e48b41c7b9ef940a296ef9a0251448d81683/src/richie/apps/core/templates/richie/base.html#L61-L67

After migrating to Kubernetes we have now the static assets and also the media on S3 Buckets. We should give the operator a way to improve performance like, give a list of hostname to prefetch the DNS and pre-connect.

This should be replaced by a list of href's to preconnect (with crossorigin) and another list of href's to dns-prefetch.

[igobranco]

@Tiago-Salles we need to implement this upstream. It is just a performance improvement.