Add frontend URL validation for uploading metrics images (edit modal)

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What does this MR do and why?

Add frontend URL validation for uploading metrics images (edit modal).

Combined add/edit metrics image dialogs into a single component since they have almost identical templates and logic.

Update related spec files.

Update pot file.

Closes: #455231 (closed)

Changelog: changed

Screenshots or screen recordings

Before	After
before.mov	after.mov

How to set up and validate locally

1. Run GitLab instance or gdk. Premium/Ultimate license is required.
2. Go to <your-project> > Monitor > Incidents. Create an incident.
3. Go to a created incident. Open Metrics tab.
4. Try to upload and then edit an incident image. Review result.