Fix: page reloading (#743).

When pressing Reload the Webclient initially selects the workspace that corresponds to the URL's topicmap reference (/topicmap/{id}).

Messages like this should not occur anymore:
{{{
TopicmapsError: topicmap 2899 not found in model (workspace_id=1615)
}}}

Workspaces REST API

New request:
{{{
GET /workspace/object/{id}
}}}
Returns the workspace a topic or association is assigned to.
Response is a workspace topic.

See #592.
See #743.