Provide a way to add / remove collections from communities

[kshepherd]

Description

The Community hibernate model and spring service provide ways to add and remove collections, but these operations are not exposed over the REST API, so there is no way to provide a "move collection" feature in the UI or with a REST integration.

Proposed solution

Allow POST or DELETE to the /collections path on the community endpoint with simple uri-list of collection URIs to add or delete?

We might have to consider what happens if we accidentally remove a collection from its only community, or if we even allow that.

[tdonohue]

This seems to be related to DSpace/dspace-angular#3196 In other words, it's probably required if we want to have a way to move Collections between Communities.

However, I'd argue we may want to consider implementing this similar to how "Move Item" works: https://github.com/DSpace/RestContract/blob/main/items.md#owning-collection

To Move an Item you are doing a PUT /api/core/items/<:uuid>/owningCollection to update the "owningCollection".

So, to move a Collection, you might do something like PUT /api/core/collections/<:uuid>/owningCommunity.

I realize "owningCommunity" is a new concept, but I think this would be safer than having separate POST/DELETE options. As @kshepherd noted above, the risk of POST/DELETE is that you might remove a Collection from a Community without linking it up to a new Community. That would currently leave the Collection "unfindable" (at least not findable through browse)