`add_emoji_by_admins_only` backend setting is represented by page_param's
`realm_add_emoji_by_admins_only` attribute. When this setting was changed
we were wrongly updating the `add_emoji_by_admins_only` attribute which
doesn't exist.
This system hasn't been in active use for several years, and had some
problems with it's design. So it makes sense to just remove it to declutter
the codebase.
Fixes#5655.
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.
