Add accessibility report as a JobArtifact

Part of #211389 (closed)

This feature will be behind a feature flag called accessibility_report_view.

What does this MR do?

This MR is the first step toward introducing Accessibility Report comparison with our Merge Request Widget.

The entire feature has been developed on an MVC branch, this MR aims to break it down in smaller chunk.

Why are we doing this?

In order to generate a diff between master and a branch on an MR we need to load our accessibility report at different levels (Build, Pipeline and MergeRequest).

As you can see with our different report type we support today, this is the first step required to introduce a new report type as a JobArtifact, similar to what we did when introducing our coverage report one month ago.

I will follow up with another MR to parse our accessibility report which will allow us to load this data for a Build and a Pipeline.

Screenshots

You can see how the feature will look like with the screenshot provided below:

Does this MR meet the acceptance criteria?

Conformity

• [-] Changelog entry
• [-] Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• [-] Separation of EE specific content

Availability and Testing

• [-] Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• [-] Tested in all supported browsers
• [-] Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• [-] Label as security and @ mention @gitlab-com/gl-security/appsec
• [-] The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• [-] Security reports checked/validated by a reviewer from the AppSec team