New Resource for Project-level notification settings

[jshcmpbll]

Allow for a default notification level to be set for a project to reduce email spam and allow for users to opt into projects they want to be notified about.

[armsnyder]

API reference if anybody wants to add a new gitlab_project_notification_settings resource: https://docs.gitlab.com/ee/api/notification_settings.html#group--project-level-notification-settings

or gitlab_global_notification_settings : https://docs.gitlab.com/ee/api/notification_settings.html#global-notification-settings