Workspace service: add getAssignedTopics() (#592).

The Workspaces service has a new method:
{{{
ResultList<RelatedTopic> getAssignedTopics(long workspaceId, String typeUri)
}}}
Returns all topics of a given type that are assigned to a given workspace.

REST API:
{{{
GET /workspace/{id}/topics/{type_uri}
}}}
Append `?include_childs=true` to include the topics child topics in the response.

Needed for "#592: workspace should have configurable ACL attributes".

See #592.