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

fix group members api doc: remove emial

"Email" is removed in ae564c97

See merge request !4232

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

Add README for updating .gitignore templates

Add notes to make it clear that users should not file a MR against a specific file here.

See merge request !4227

fix user.name length in milestones participants tab

Spelling mistake of "length"

See merge request !4206

Fix fork counter ui bug for safari

## What does this MR do?

Fixes UI bug on safari when fork button has blue arrow

## Screenshots 

![Screenshot_2016-05-19_22.10.21](/uploads/d923d8fc6bc12ee8f7052390bd5d7c51/Screenshot_2016-05-19_22.10.21.png)![Screenshot_2016-05-19_22.09.38](/uploads/130eb8b4a09b2ba5b7c5a83fed543dbf/Screenshot_2016-05-19_22.09.38.png)

See merge request !4221

Issuable form due date

For some reason, this was never in the original merge request for due date?

![Screen_Shot_2016-04-22_at_21.49.44](/uploads/7d4e2b07f75a33009e3170fdb076a4bd/Screen_Shot_2016-04-22_at_21.49.44.png)

Closes #15516

See merge request !3895