Either organize spaces in folders, or sub-spaces

The left navigation colum currently is limited to showing all spaces, or to hide them under ‘hidden spaces’.

This already is a UX limitation, since we often don’t want to hide it, but simply organize the structure of spaces better. Folders for spaces would be an outcome.

Ideally, but that may be more complex (?) we could allow spaces to be part of spaces (creating sub-spaces). This would be nice for better permissions inheritance also.