Reflect connection disabled/enabled in new REST API

[pmeulen]

A connection can be disabled in the admin tab. This currently has no effect in the new (and the old) REST API. The intended effect of disabling a connection is that it is like a deleted or non-existent connection to consumers of the API.

I see no use case for exposing enabling/disabling connection or querying disabled connections though the API now, though we might want to add that in a future version.