WS API: config for disabling release notifications

[Kit Randel]

Websocket API support is required for storing configuration for disabling release notifications.

design: https://app.zeplin.io/project/5efa65801b4d9162739308fc/screen/5f16b3b5311acb77fa0925d7
spec: https://docs.google.com/document/d/1kxeY_l7ZXjLa2Aq3higbDGpPCF52B15IRs9hheJ6FWM/edit#heading=h.oncww1139tq8

[Lee Trager (ltrager)]

According to this branch[1] and the design this is done as a MAAS setting. You should be able to use the existing ConfigHnalder to modify this value.

[1] https://code.launchpad.net/~d0ugal/maas/+git/maas/+merge/388137

[Lee Trager (ltrager)]

As discussed on MM I'm assigning this to Dougal as hes already working on adding this as a config option.

[Kit Randel (blr)]

Thanks Lee