Allow anonymous user to access pipelines

## What does this MR do?

It fixes an issue where the Pipelines is shown for the Anonymous users,
but they get 404 when clicked. Their session is then logged out.

Fixes #17717.

See merge request !4233

[ci skip]

[ci skip]

Fix MySQL compatibility in zero downtime migration helpers

## What does this MR do?

This MR fixes MySQL for zero downtime migration helpers introduced in https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/3860

Closes #17711

See merge request !4239

Fix the CI login to Container Registry (the gitlab-ci-token user)

## What does this MR do?

This fixes `docker login` not succeeding when trying to do CI login: `gitlab-ci-token with $CI_BUILD_TOKEN`.

cc @marin

See merge request !4236

Add health check feature documentation

for https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/3888

@virtuacreative @axil 

See merge request !4199

Fix docker registry integration

See merge request !4229

[ci skip]

Geo: Single Sign Out

Implements Single Sign Out for Geo (#76).

Initial proposal was to generate a hash based on the `access_token`, but that created a O(N) cost against a desirable O(1), as a new `access_token` is generated for each new login. To overcome that cost we would need to send a "public identifier" to help retrieve the correct `access_token` and provide that during login process.

This is also how most Single Sign On implementations works (they provide some sort of session_id, that we notify every node to invalidate, during sign out process).

As I don't want to modify our OAuth table (that is managed by doorkeeper) nor change the way our login process work, the solution is to encrypt the `access_token` using a symmetric key known by both nodes, and expire the `access_token` after the logout to prevent replay attacks (otherwise we would need to send a `nounce` and store that on primary). 

The key is based on `Gitlab::Application.secrets.db_key_base` which we already use to encrypt database attributes and is synced between both nodes. We communicate sending a `state` parameter which is known terminology in OAuth protocol.

Although this is implemented with Geo only in mind, we can backport to CE (with minimal changes) and provide as a "non-standard" way of single sign off for applications that integrate with GitLab.

Fixes #522 

See merge request !380

# Conflicts:
#	app/services/system_note_service.rb

Fix Error 500 when attempting to retrieve project license when HEAD points to non-existent ref

Closes #17537

See merge request !4151

Fix group visibility level migration in case all visibility levels are restricted

fixes #14632 

See merge request !4107

Let users set notification levels in projects which they are not members

Fixes #9013 

See merge request !3986

Implement special GitLab markdown reference for milestones

https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/3453 by @eReGeBe, with two additions:

- Move changelog item to 8.8
- Fix cross-project milestone ref with invalid project, like https://gitlab.com/gitlab-org/gitlab-ce/commit/f7348cd348ad8f4a18d74dd668283a4e236f5790 did for labels

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/13829

See merge request !3897

current_user.confirmed_at.present? => current_user.confirmed?

fixes: https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/3862/diffs#4f7b76b1197d6fb073f77957b3b7b70d921ed7e7_42_41

See merge request !4225

Allows MR authors to have the source branch removed when merging the MR


closes #13191 

The location of the checkbox might not be optimal so any feedback is welcome. Any other feedback too obviously.

Screenshot:
![Screenshot_2016-02-17_21.25.24](/uploads/a9c3eaafb39c6f5b4f0949a2278af6da/Screenshot_2016-02-17_21.25.24.png)

See merge request !2801

Update CE code to include some refactor done in EE to do with import url

This is simply updating the code to match EE and avoid further conflicts related to `import_data` and `import_url` changes made on EE only.

See merge request !4223

This wasn't removed when merging CE into this branch.

Updated webhooks UI

The webhooks files are in different places for EE & CE so this was missed when merging CE into EE.

## Group UI

![Screen_Shot_2016-05-17_at_10.05.39](/uploads/6e2942e478b574545b45989365307c24/Screen_Shot_2016-05-17_at_10.05.39.png)

## Project UI

![Screen_Shot_2016-05-17_at_10.06.08](/uploads/61380b40568bbc260932895be83d6bae/Screen_Shot_2016-05-17_at_10.06.08.png)

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/17525

See merge request !404