Customize notifications for new epic - Web and API

Follow up https://gitlab.com/gitlab-org/gitlab-ee/issues/5480 to update the API in relation to epic notifications, to be consistent with issues and merge requests.

When you have custom notifications for a group, you have these settings:

We should have the same thing for the New epic event.

It should be in this same modal for customizing group level notifications, and should be exposed in the API, i.e. https://docs.gitlab.com/ee/api/notification_settings.html