Display success message when model saved/edited

What does this MR do and why?

Displays a success alert upon page redirect when a self-hosted model has been successfully created or edited.

MR acceptance checklist

Please evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

Screenshots or screen recordings

Model Created	Model Edited

Screen Recording Screen_Recording_2024-10-09_at_6.20.32_PM

How to set up and validate locally

Prequisites

You will need an active GitLab ultimate license.

1. Start GDK instance in self-managed mode:

GITLAB_SIMULATE_SAAS=0 && gdk start

2. In the rails console, enable Code Suggestions and the custom models experiment:

Feature.enable(:ai_custom_model)
Feature.enable(:custom_models_vue_app)
Feature.enable(:custom_models_feature_settings_vue_app)
ApplicationSetting.first.update(duo_features_enabled: true)

Test it!

1. Log in as an admin
2. Visit the page Admin Area > AI-powered Features > Self-hosted models
   
3. Create a self-hosted model. Upon successful creation you should be redirected to the self-hosted model index page and a success alert should appear.
4. Edit a self-hosted model. Upon successful creation you should be redirected to the self-hosted model index page and a success alert should appear.

Related to #476662 (closed)