Added a dropdown in the organization settings page with a search-box and
required styles. Also added an element to disable it. Added a method to
populate the dropdown using list_rendering.js. Also altered response to
the event of deletion of the notifications stream on the frontend. On
selection of a new stream or on clicking 'Disable', a patch request is
made with stream-id to /json/realm.
Fixes: #3708.
Akhil