Related resource integration

[juliusknorr]

Collectives is not yet integrated with https://github.com/nextcloud/related_resources.

As far as I see it would require implementing:

• Adding the NcRelatedResourcePanel to collectives
• Implement a provider, but this is currently not possible in the app owning the resources https://github.com/nextcloud/related_resources/tree/master/lib/RelatedResourceProviders