Spaces: API attribute "recommended" to sync Spaces to local disk by default

## Is your feature request related to a problem? Please describe.

Majority of regular users, especially ones connected to small and medium instances, expect to sync all their Spaces to their desktop computer by default. (no additional user interaction required). With VFS behaviour in desktop and mobile apps, local disk space isn't a problem.

But there are more pro-usecases, with thousands of available Spaces for a user, and there user expectation is maybe, to individually decide to sync a Space or not.

## Describe the solution you'd like

To deal with this, Spaces API could get an additional attribute "recommended", that means sync clients sync all those Spaces to the local disk by default.

In the first iteration, this attribute could be hardcoded – no backend logic needed.

In a future iteration, admin users and Space owners should be able to set, if a Space is "recommended" for all members, or only "recommended" for user/groups.