blackout - make service editable

[m4ce]

@satterly - we have been using the blackout feature quite a bit now. It's been working really well. One problem we identified is that we cannot create blackouts for services that alerta has no knowledge of. The issue is that once the housekeeping runs, it will also purge information about services. For this, we had to fall back to the command line to be able to specify a custom service. I am not sure if there's an easy way of allowing for that in the UI