Skip import when source and target path are the same

Problem/Motivation

Currently, it's possible with this module to import a redirect where the source and target path are the same.
When you import this kind of redirects, this will result in a "Service unavailable" error, since redirect recognizes this as a recurring redirect.

Steps to reproduce

1. Enable this module and visit the import form.
2. Upload this CSV: migrate.csv → and start an import.
3. Visit the URL /my-redirect-url on your site. You now receive a "Service unavailable error.

Proposed resolution

Add a check, when source and target url are the same, skip this row.

Remaining tasks

User interface changes

API changes

Data model changes