This commit adds a new sidebar item to the group security dashboard.

It consists of an accordion, showing projects grouped by the
most severe vulnerability within the project.

This commit swaps the line chart with a series of sparkline charts
and additional information (severity level, count of current
vulnerabilities per level)

This commit replaces the currently used `<main>` with an
`<article>` to ensure component re-usability.

(Using a main tag within a vue-component introduces the possibility
of having two or more main tags on a page, this causes accessbility
issues)

This commit addresses some UX changes to improve the group security
dashboard:

* Moves the vulnerabilities chart into an aside
* Changes some elements to to enhance markup-semantics
* Hides vulnerabilities count
* Makes chart buttons span full width
* Adds date information
* Removes labels from charts
* Changes chart layout to make use of free space
* Updates documentation to reflect changes

Adds an EE override of groups#show action that
renders groups/security/dashboard#show view and
substitutes the controller_path, controller_name
and action_name to pretend that we're still
rendering groups/show for JS bundles and sidebar
navlinks to display and load correctly