Fix description issues with custom roles

What does this MR do and why?

This MR fixes several things:

1. On the Admin -> Settings -> Roles and Permissions page, when trying to edit a custom role that does not have a description, the edit page does not show the edit form, and will endlessly show the loading spinner. This has been fixed:

Roles and Permissions page	Before	After

2. On the Create role page, the role dropdown was changed from a GlFormSelect to a GlCollapsibleListbox. This fixes the "no placeholder text and first item pre-checked" issue in #478667 (closed):

Before (no placeholder text)	Before (first item pre-checked)	After

3. The terms Static roles and Standard roles were renamed to Default roles, as per the feedback from ~"Technical Writing". This affects the category name for the role dropdown in the invite modal and role details drawer:

Invite modal

Before	After

Role details drawer

Before	After

4. Previously, in the role dropdown, custom roles that did not have a description, did not show the role description text. Now, it will show No description in a slightly more faded gray:

Invite modal

Before	After

Role details drawer

Before	After

How to set up and validate locally

Follow this video walkthrough (with audio commentary) on how to validate locally:

2024-08-28_16-12-20

Related to #481410 (closed), #473342 (closed), #478667 (closed)