Resolve "User-generated permalink IDs collide with GitLab interface"

## What does this MR do?

Prevents ID values automatically generated by headers in [GitLab Flavored Markdown](https://github.com/gitlabhq/gitlabhq/blob/master/doc/user/markdown.md#header-ids-and-links) from colliding with IDs used elsewhere in the GitLab interface.  This can cause confusion when, for instance, a selector looks for a merge request tab with `id="pipelines"` and there is a header with the same ID earlier in the DOM.

How this works:
* All header IDs generated with GitLab Flavored Markdown are namespaced with `id="user-content_foo"`
* All anchor links which point to these IDs continue to use the non-namespaced hash `<a href="#foo">...</a>`
* When a page is loaded or when the `hashchange` event is triggered, javascript will automatically search for `#user-content_foo` if `#foo` cannot be found, and scroll to that position instead.

## Before

![2016-11-21-13.00.28](/uploads/e3be2cd6a9142dfd6e64db5462a6aa76/2016-11-21-13.00.28.gif)

## After:

![2016-11-21-13.12.45](/uploads/f7ae3f3a30c91325eaa3665591b6a850/2016-11-21-13.12.45.gif)  
![2016-11-21-13.03.00](/uploads/3a6a782c081ecaa05b8781548d794909/2016-11-21-13.03.00.gif)  

## Does this MR meet the acceptance criteria?

- [x] [Changelog entry](https://docs.gitlab.com/ce/development/changelog.html) added
- Tests
  - [x] Added for this feature/bug
  - [ ] All builds are passing
- [x] Conform by the [merge request performance guides](http://docs.gitlab.com/ce/development/merge_request_performance_guidelines.html)
- [x] Conform by the [style guides](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CONTRIBUTING.md#style-guides)
- [x] Branch has no merge conflicts with `master` (if it does - rebase it please)
- [x] [Squashed related commits together](https://git-scm.com/book/en/Git-Tools-Rewriting-History#Squashing-Commits)

## What are the relevant issue numbers?

Closes #22781 

See also prior attempts to address this issue:
#3908, !2023, !2024


See merge request !7631

New `gitlab:workhorse:install` rake task

## Why was this MR needed?

Because with this we can remove the "Ensure the gitlab-workhorse version in Install gitlab-workhorse matches the required version." step from https://gitlab.com/gitlab-org/release-tools/blob/master/doc/release-candidates.md#creating-rc1! MR is ready: gitlab-org/release-tools!57

Closes #22719

See merge request !6574

Signed-off-by: Rémy Coutable <remy@rymai.me>

Signed-off-by: Rémy Coutable <remy@rymai.me>

This is the Group equivalent of 13ad9a745a392e0bf0cedd0e1f318c1acee9b969

Create dynamic fixture for build_spec

## What does this MR do?

Replace `spec/javascripts/fixtures/build.html.haml` by a dynamically created fixture (using `rake teaspoon:fixtures`).

## Why was this MR needed?

The existing fixture was not representing the real page. 

## What are the relevant issue numbers?

#24614 would have been avoided

following !6059

See merge request !7589

Pass `--load-images=no` to PhantomJS via Capybara/Poltergeist

We were unintentionally hitting `gravatar.com` whenever a test that used
Poltergeist was run. This was certainly wasting their resources and
slowing down our tests even further, for no reason.

Part of https://gitlab.com/gitlab-org/gitlab-ce/issues/24899

See merge request !7781