Added remove due date button

In the sidebar when there is a due date a link to remove due date becomes visible

![remove-due-date](/uploads/126baa61f5ec44ce503ce374d5331ba6/remove-due-date.gif)

Closes #17392

See merge request !4209

In the sidebar when there is a due date, a link to remove due date becomes visible

Closes #17392

Fix Error 500 when accessing application settings due to nil disabled OAuth sign-in sources

## What does this MR do?

When upgrading to GitLab 8.8, an admin would encounter an Error 500 due to a `nil` `disabled_oauth_sign_in_sources`. This MR ensures that the value will be a blank array if empty.

Closes #17564

See merge request !4242

[ci skip]

[ci skip]

Fix some specs which would have broken in 2017 to not break anymore by using a dynamic year value.

Static year values would have caused the test suite to fail on these in the future, so I replaced 'em.

That's pretty much all there is to this MR.

See merge request !4241

Docker container registry configuration docs.

Part of gitlab-org/omnibus-gitlab!764 and gitlab-org/gitlab-ce!4040

See merge request !4141

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

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

Upgrade activerecord-session_store from 0.1.2 to 1.0.0.

Changelog: https://github.com/rails/activerecord-session_store/releases/tag/v1.0.0

Includes Rails 5 support, working towards the never-ending #14286.

See merge request !4238

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