for now leave out type `grant` and `mountpoint` when listing spaces

For the beta, a call to `/drives` should not list drives of type `grant` or `mountpoint`. The only drives in the list will be the users `personal` drive (same owner as logged in user), the share jail (type `virtual`) and multiple `project` spaces.

The `grant` and `mountpoint` split of a share is a detail that we will move out of reva, by implementing a stateful `/me/drives` endpoint. reva will know about `grant` spaces and users can set a personalized space alias in a future drives service via the `/me/drives` endpoint. This can wait until after the GA, though.

Clients can access individual shares using the webdav url of the share jail and the share name, eg. `http://demo.owncloud.com/dav/spaces/{sharejailid}/{sharename}`.

After GA we will prepare an ADR that describes how we want to expose individual shares as spaces and how clients will get notified of new shares.

Related Issues:
- https://github.com/owncloud/ocis/issues/3425