Enhancement: Add a member to a space

A new option to add members to a space has been added. It will be only visible for users with proper permissions.

https://github.com/owncloud/android/issues/4613
https://github.com/owncloud/android/issues/4740
https://github.com/owncloud/android/issues/4770
https://github.com/owncloud/android/pull/4754
https://github.com/owncloud/android/pull/4764
https://github.com/owncloud/android/pull/4777
https://github.com/owncloud/android/pull/4847
