AddGrant and UpdateGrant should be SetGrant

[butonic]

While the usecase of limiting a user to only add grants seems valid, it already comes with the requirement to let him manage the grants he created, which requires UpdateGrant permissions.

Having a single permissions for a SetGrant would be sufficient. If users should be limited to manage their own, all or only the permissions for the groups they are a member of is implementation specific and would require the share owner to be part of the Grant, which it currently is not. Currently, a resource can only be owned by a single User