Refactor workspace creation (#592).

Workspace API

`createWorkspace()` is RESTful:
{{{
POST /workspace/{name}/{uri}/{sharing_mode_uri}
}}}
The URI is optional: the `uri` path segment may be empty.

new REST-client method:
{{{
dm4c.restc.create_workspace(name, uri, sharing_mode_uri)
}}}

In preparation of fixing the missed System workspace default topicmap.

See #592.