Store workspace assignment as property (#668).

Thus a topic's/association's workspace assignment can be determined without fetching the topic/association in advance.

BREAKING CHANGES

Workspace API

1 method has changed:
{{{
Topic getAssignedWorkspace(long id)
}}}
It takes a topic/association ID instead of a `DeepaMehtaObject`.

1 method is added:
{{{
Topic createWorkspace(String name, String uri, String workspaceTypeUri)
}}}
So, the workspace type URI must be passed when creating a workspace.
The former 2 `createWorkspace()` methods are dropped.

See #668.