Fix emoji aliases not showing in autocomplete

closes #14948 

See merge request !3595

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

Add proper nil and error handling to SAML login process

While writing the feature that would allow certain Omniauth providers to be marked as external I noticed that there is a scenario where the `gl_user` method can return `nil` and if this is not properly checked, it will lead to exceptions that will cause 500 errors. It is quite easy to land in this scenario, so I added `nil` checks.

I also noticed that the `saml` method in the `omniauth_callbacks_controller.rb` file lacked a `rescue` for `Gitlab::OAuth::SignupDisabledError`, which can happen if the default configuration from `1_settings.rb` is applied. So I also added this check.

See merge request !3609

Delete notes via API

Supports deleting issues, snippets, and merge requests via the API.

* Closes #14944
* Closes #14845 
* Closes #6060 

@zj I did not see that you assigned yourself in #6060. Hopefully, you did not start yet.

@rymai In #6060 this is targeted for 8.7 release. Could you review that and maybe this still lands in 8.7. 


See merge request !3557

API: Ability to retrieve a single tag

* Closes #14330

/cc @dblessing

See merge request !3577