Add custom role details tab to role details page

On the Admin Area -> Settings -> Roles and Permissions page, a table of custom roles is shown:

In a previous MR, we added a new role details page that this table will link to when a role is clicked on. The details page was just a placeholder that showed the role name and action buttons. This MR adds the role details tab to the page, for both standard roles and custom roles:

Standard role:

Before	After

Custom role:

Before	After

How to set up and validate locally

For self-managed, follow this video with audio commentary:

2024-08-12_15-06-29

For SaaS, first run this bash command to enable Saas mode, then restart GDK:

export GITLAB_SIMULATE_SAAS=1
gdk restart

Then follow this video with audio commentary:

2024-08-12_15-09-14

Related to #468386 (closed)