Improve notification settings for subgroups

Notifications settings should cascade from project, through each group, and then to the global setting if “Global” is set on the project or any of the groups

• if user has project notification settings - use those.
• If it defaults to Global then look for parent notification settings.
• If parent defaults to Global too, then look one level up and up until we find something.

So basically same behavior like we had before with project/group but with possible N subgroups in between.

See https://gitlab.com/gitlab-org/gitlab-ce/issues/30549#note_28268440 comment for further details.