Currently this works by loading the HAML partials via XHR. While this is
not the nicest setup it _is_ the easiest setup using the tools we
currently have.

Loading this data asynchronously doesn't make loading the related
MRs/branches itself faster, it merely ensures that loading the issue
itself is not slowed down.

Fixes gitlab-org/gitlab-ce#14949

Move RepositoryArchiveCacheWorker to sidekiq-cron

Closes #15105

See merge request !3663

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

Build notification null check

Cancels build notification interval on page change

See merge request !3540

Closes #15105