Fix creating a user account (#751).

The original problem of User Account Management is now solved.
- The admin can edit hers user account (and see no other user accounts).
- The same applies for all user accounts created interactively.
- After creating a user account its Username topic (which is public) is revealed in the topicmap,
  ready for being associated with workspaces (via Membership).

Issues:
- The user who creates a user account becomes (by default) a member of the new user's private workspace.
  While annoying this is not a pressing problem as memberships of private workspaces are ignored.

Changes in Access Control API:

The topic returned by the `createUserAccount()` call is the account's **Username** topic (not the User Account topic).
Note: the ''User Account'' topic is private and thus not accessible by the user who made the `createUserAccount()` call.
{{{
Topic createUserAccount(Credentials cred)
}}}

1 new method in interface `de.deepamehta.core.service.accesscontrol.AccessControl`:
{{{
void assignToWorkspace(DeepaMehtaObject object, long workspaceId)
}}}
It creates a workspace assignment while bypassing access control.

See #751.