Create UI to list and create custom roles in self-managed

What does this MR do and why?

New UI to list and create custom (member) roles. The UI is based on personal access token UI, which was updated recently.

We turned on the custom_roles_ui_self_managed feature flag. This flag was used for incremental development. Instead of removing it, we turned it on, just in case it causes problems.

Changelog: changed

EE: true

Relates: Create new custom role in UI (#393235 - closed)

Screenshots or screen recordings

Screen_Recording_2023-08-10_at_13.11.46

How to set up and validate locally

1. Enable this feature flag: echo "Feature.enable(:custom_roles_ui_self_managed)" | gdk rails console

2. Enable the Ultimate license in one of the groups. This is done going to /admin/groups select Edit on a top level group, and choose the Ultimate plan. For example, /admin/groups/flightjs/edit:

2. Go to /admin/application_settings/roles_and_permissions, and play creating custom roles for the group in step #1.