Make custom notifications more flexible

With https://gitlab.com/gitlab-org/gitlab-ce/issues/24892 and https://gitlab.com/gitlab-org/gitlab-ce/issues/33620 done, we can start to rethink our custom notification settings.

I would like to have this as the end goal:

  • Every existing notification level is possible with custom notifications.
    • Currently, this is only possible for Participate, with no custom events checked. I imagine allowing people to set 'Mention' and 'Participate', for instance, so that a custom notification with nothing set is the same as having notification settings disabled.
  • The Watch level currently does not include all custom notifications - because we skip successful pipelines unless you're specifically subscribed to them, but we should be able to add more events to this, and make it clear which are included at each level.

This is the current modal:

image

Related issues: