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.