Make incident modal interactive and create incident

What does this MR do and why?

This MR adds interactivity to the modal and creates incidents when one submits the modal form.

You will get an incident issue link when the modal is submitted successfully and the incident is created.

This feature is behind a feature flag and is disabled by default.

Screenshots or screen recordings

Part 1: Part_1

Part 2: Part_2

How to set up and validate locally

1. Setup slack dev workspace using https://gitlab.com/gitlab-org/manage/integrations/team/-/blob/main/integrations/slack.md
2. Switch to this feature branch on GitPod https://gitlab.com/gitlab-org/manage/integrations/team/-/blob/main/integrations/slack.md#switching-to-your-feature-branch
3. On the GitPod terminal, restart GDK: cd /workspace/gitlab-development-kit && gdk restart
4. Make sure you visit the GitPod instance site in your browser so the development environment loads, otherwise you may experience timeouts in Slack.
5. Enable the feature flag - incident_declare_slash_command.
6. After setup, create a slack integration user using /dev-gitlab <any project alias where integration is enabled> issue show 1.
7. Start the modal using /dev-gitlab incident declare and play around the modal for usage.

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 #344856 (closed)