Cascading notification settings

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.