Add frontend alert for webhook disabled alert

What does this MR do and why?

This feature is behind the feature flag webhooks_failed_callout (rollout issue).

This adds the frontend code for a project-wide alert (visible to owner / maintainer) when a webhook fails for some reason. Once a user dismisses it, it should not show again (persistent user callout functionality).

Screenshots or screen recordings

How to set up and validate locally

1. In rails console enable the callout feature flag.

   Feature.enable(:webhooks_failed_callout)

2. In rails console enable the general feature flag.

   Feature.enable(:web_hooks_disable_failed)

3. Make sure you are signed in with a project owner / maintainer account.
4. Go to Project > Settings > Webhooks and create a dummy webhook. For example, http://127.0.0.1:3000/gitlab-org/gitlab-shell/-/hooks.
5. In Project Hooks, click Test which should fail (as long as the URL above was fake).
6. Go to any project page and observe the alert shown.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related to #363027 (closed)