Propfinds in spaces trash-bin don't contain permissions

## Is your feature request related to a problem? Please describe.
@JanAckermann noticed that PROPFIND responses for spaces trash-bins don't contain the permission attribute.
The web client needs to somehow check if the current user is allowed to purge deleted items or not.
Our current understanding is that space managers can purge items and all other member roles can not.

## Describe the solution you'd like
When the PROPFIND response would contain the permission attribute we could set the permissions according to the space member role. Based on this attribute the client could then decide to show the "Empty Trashbin" button or not.

## Describe alternatives you've considered
An alternative to the PROPFIND response attribute would be to somehow map the space member role to permissions and gather from that if the user can purge deleted items.

@micbar @butonic, any ideas?