Add custom role details page

What does this MR do and why?

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

We are adding a new page to show the custom role details when a role is clicked on in the table. This MR adds in the custom role details page. For now, just the header is shown, which shows the role name, header message, and action buttons:

Additional role details will be added in a follow-up MR. The page also supports showing built-in standard roles, in which case a different header message is shown.

How to set up and validate locally

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

2024-08-06_10-43-02

For SaaS, first enable the GITLAB_SIMULATE_SAAS environment variable and restart GDK:

export GITLAB_SIMULATE_SAAS=1

Then follow this video walkthrough (with audio commentary):

2024-08-06_10-49-01

Related to #468386 (closed)