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

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

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.