Fix credential check (#751).

Checking the credentials performs with special privilegs (inside the Core).

Set ownership for private workspaces and for the "System" workspace.

NOTE: creating user accounts does not yet work.

Access Control API:

1 new method in interface `de.deepamehta.core.service.accesscontrol.AccessControl`:
{{{
boolean checkCredentials(Credentials cred)
}}}

BREAKING CHANGES

Access Control API:

1 renamed method:
{{{
Topic getUsername(String username) -> Topic getUsernameTopic(String username)
}}}

Class `Credentials` moved package:
{{{
de.deepamehta.plugins.accesscontrol.model -> de.deepamehta.core.service.accesscontrol
}}}

See #751.