Draft: Save previous route and fallback to index page otherwise

What does this MR do and why?

Fix bug when going to the Trigger Edit page directly and submitting the form

References

Screenshots or screen recordings

Before	After

How to set up and validate locally

What happens:

1. Go to http://gdk.test:3000/gitlab-duo/test/-/automate/flow-triggers
2. Click on edit on any flow trigger
3. Open the same URL in another tab
4. Click the Save button
5. You are redirected back to your browser default page

What should happen:

1. Go to http://gdk.test:3000/gitlab-duo/test/-/automate/flow-triggers
2. Click on edit on any flow trigger
3. Open the same URL in another tab
4. Click the Save button
5. You are redirected back OR fallback to the flow trigger overview

MR acceptance checklist

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