Migrate `app/views/admin/users/_modals.html.haml` modal implementation to `GlModal`

The markup in app/views/admin/users/_modals.html.haml replicates that of GlModal. The implementation in app/assets/javascripts/admin/users/components/modals/user_modal_manager.vue reads the necessary attributes from the markup and uses them with GlModal component. In the scope of this issue migrate the template to Vue and simplify the implementation of the UserModalManager.

Example - admin delete user modal

This partial is referenced in a couple of other places

grep 'admin/users/modals' -lr .

1. ./app/views/admin/impersonation_tokens/index.html.haml
2. ./app/views/admin/users/show.html.haml
3. ./app/views/admin/users/keys.html.haml
4. ./app/views/admin/users/projects.html.haml
5. ./app/views/admin/users/_users.html.haml
6. ./app/views/admin/identities/index.html.haml

⚠ There is already an issue for this migration and Manage::Compliance team has some ideas on how to migrate these modal instances. Please, collaborate with @jiaan in case you want to work on this.