Using some stored workspace as a "default workspace"

... for users who get a newly created workspace.

If the Setting 'default_workspace_anonymous_user' exists and is the
secret slug of some existing stored workspace, then the items in it
will be copied to any anonymous user's workspace when it is first
created.

The same holds for 'default_workspace_user' and logged in users,
although it is probably less useful for them since their workspace is
only initially created once, after that they keep using their existing
one.