GitLab pages: Stricter validation of custom page domain

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What does this MR do and why?

• Related to #467235
• Extending the test suite to cover more domain name entries
• Ensuring that only valid top-level domain names are accepted
• Adding an alert

🛠 with ❤ at Siemens

MR acceptance checklist

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

MR Checklist (@gerardo-navarro)

• Changelog entry added, if necessary
• Documentation created/updated via this MR
• Documentation reviewed by technical writer or follow-up review issue created
• Tests added for this feature/bug
• Tested in all supported browsers
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the javascript style guides
• Conforms to the database guides

Screenshots or screen recordings

Before	After
example.invalid-tld was accepted
domain name example (without the tld .com)

How to set up and validate locally

1. Go to the page for creating a new domain: http://gdk.test:3000/flightjs/Flight/pages/domains/new
2. Try out different domain names, e.g. example, example.invalid-tld, https://domain-name-with-protocol-information.com, etc.