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: NDmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>

See merge request !3421

Prefills commit message in edit file

See #14488 

![Screen_Shot_2016-04-06_at_10.18.06](/uploads/2ace2a21a6ead8d83c79ef9ce5dd34cc/Screen_Shot_2016-04-06_at_10.18.06.png)

See merge request !3564

Build notification null check

Cancels build notification interval on page change

See merge request !3540

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

Fix #14753: Check if head is born before trying to detect main language

This MR makes sure that head exists before trying to detect the main language.
This prevents errors on repo's without a master branch.

Closes #14753

See merge request !3654

Instrument all service classes

This will help us see where (mostly) Sidekiq code is spending time.

See merge request !3675

Hide help block when user is creating a new project inside a group

closes #14092 

![fix](/uploads/13721117c410c8fe6937d44a3b0a90a8/fix.gif)

See merge request !3427

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

[ci skip]

API: Ability to retrieve a single tag

* Closes #14330

/cc @dblessing

See merge request !3577

Allow a project member to leave the projected through the API

Closes #7687 


See merge request !3613

Un-instrument Banzai::ReferenceExtractor

No changelog entry since this isn't really something new compared to 8.6, instead it adjusts something that was added for 8.7.

See merge request !3676

Instrumenting this class together with Gitlab::ReferenceExtractor causes
a StackError for some reason. Since Gitlab::ReferenceExtractor has most
of the interesting code we'll only instrument that class.

API: Add iid filter to milestones

This was documented in the API docs but it was never working because the iid filter was never applied to the milestones. The tests were working by accident because the were checking the first element, which was by accident the request element. 

* Closes https://github.com/gitlabhq/gitlabhq/issues/10122

See merge request !3588

API: Expose updated_at for notes

* Closes #7954

See merge request !3560

Fixes gitlab-org/gitlab-ce#15162

API: Expose open_issues_count, closed_issues_count, open_merge_requests_count on labels

Partly fixes https://gitlab.com/gitlab-org/gitlab-ce/issues/13238

See merge request !3569

Fix milestone removal problem when editing issues

fixes #1752 

See merge request !3571

See #14488