Decouple membership and notifications

This allow you to have notification setting per project even if you are member of group. 
It also creates background for having notification settings in project you are not member of. 


- [x] Make it work
- [x] Migrations
- [x] CHANGELOG
- [x] More tests
- [x] API

For #3359 

After this merge request there is still some work to be done: 

* create migration that remove duplicates in notification settings table and create uniq index (8.8 probably)
* remove notification_level field from Member model in 9.0
* make proper API for notification settings
* use `MemberCreateService` instead of Member#after_create callback for creating notification settings (after #14709) 
* maybe more tests 
Signed-off-by: Dmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>

See merge request !3421