Fix Spinach merge request diff failures

gitlab-git-test `master` was updated in ff076d88, and this caused the merge request diffs to change in a way that broke assumptions in the Spinach tests.
    
Partial fix to #23378

See merge request !6916

Convert CHANGELOG to Markdown

All this does is convert the version sections into headers. The list
items shouldn't really be indented by two spaces, but it makes no
difference to the rendering and this way we retain authorship history
for the actual changes.

Related to https://gitlab.com/gitlab-org/release-tools/merge_requests/29

[ci skip]

See merge request !6925

All this does is convert the version sections into headers. The list
items shouldn't really be indented by two spaces, but it makes no
difference to the rendering and this way we retain authorship history
for the actual changes.

Related to https://gitlab.com/gitlab-org/release-tools/merge_requests/29

Add the tech writers usernames in the doc_sytleguide doc

See merge request !6924

# Conflicts:
#   doc/development/doc_styleguide.md

Signed-off-by: Dmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>

Signed-off-by: Dmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>

Fix 500 error when creating mileston from group page

Part of https://gitlab.com/gitlab-org/gitlab-ce/issues/23378

See merge request !6920

Signed-off-by: Dmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>

Fix 404 when group path has dot in the name

Fixes https://gitlab.com/gitlab-org/gitlab-ce/issues/23401

See merge request !6919

Signed-off-by: Dmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>

Added book club to university

## What does this MR do?

Adds the old University Book Club to the new University content

## What are the relevant issue numbers?

Closes #23353

See merge request !6888

[ci skip]

Includes a note on valid paths for cache and artifacts in the
place users are most likely to learn about the feature.

## Why was this MR needed?

We get regular support requests relating to cache and artifacts
not being picked up outside the project root. It's intended
behaviour, and should be documented.

## What are the relevant issue numbers?

https://gitlab.com/gitlab-org/gitlab-ci-multi-runner/issues/1792

See merge request !6892

Fix Hash syntax to work for both Ruby 2.1 and 2.3

Ruby 2.1 specs were failing (e.g. https://gitlab.com/gitlab-org/gitlab-ce/builds/5149275) due the error:

```
SyntaxError: /builds/gitlab-org/gitlab-ce/app/views/devise/shared/_tabs_normal.html.haml:3: syntax error, unexpected ':', expecting =>
...'#login-pane', data: {'toggle':'tab'}, role: 'tab')).to_s));...
```

See merge request !6917

Fix broken SCSS linter errors due to missing newlines

See merge request !6918

gitlab-git-test `master` was updated in ff076d88, and this caused
the merge request diffs to change in a way that broke assumptions in the
Spinach tests.

Partial fix to #23378

Handle unmatched routing with not_found method

## What does this MR do?

Handle all unmatched routes with 404 method

## Why was this MR needed?

We need this to prevent routing error when user access URL like /123 when there is no resource located under such name. 

## What are the relevant issue numbers?

Related to https://gitlab.com/gitlab-org/gitlab-ce/issues/23378 and https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/6733.

See merge request !6905

Improve ExtractsPath logic related to atom format

* Fixes one test from https://gitlab.com/gitlab-org/gitlab-ce/issues/23378. 
* Fixes issue introduced by https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/6750.

See merge request !6907

remove ashley since she no longer works here

## What does this MR do?

Merges gitlab-org/gitlab-ee!803 in the right place.

This was already merged in EE because I sent the MR there by accident.

See merge request !6913

Fix Spinach failure due overprecise percentage matching

The percentages in the language match changed by a tenth of a percentage point for Ruby and JavaScript, which led to this failure.
    
Partial fix to #23378

See merge request !6912

This reverts commit 8751491b.