Enhancement: Implement people sharing for spaces

Spaces can now be shared with other people. This change specifically includes:

* listing all members who have access to a space (possible for all space members)
* adding members to a space and giving them dedicated roles (possible for managers only)
* editing the role of members (possible for managers only)
* removing members from a space (possible for managers only)

https://github.com/owncloud/web/pull/6455
https://github.com/owncloud/web/pull/6572
https://github.com/owncloud/web/issues/6283
