Move Custom notification settings modal to JS

Right now on the project home page we also render a modal in HAML for the custom notifications (look for `custom-notifications-title) so this is only shown when someone clicks on the notification setting button but is loaded + rendered all the time. It even includes a couple of db queries which should help on top for the BE rendering time.

HAML File https://gitlab.com/gitlab-org/gitlab/blob/master/app/views/shared/notifications/_custom_notifications.html.haml

• We need to convert this to an API/JS async solution and not render it on every page load
• Use Pajama Components