Use Gitlab::CurrentSettings in UrlValidator

What does this MR do?

Use Gitlab::CurrentSettings in UrlValidator.

Gitlab::CurrentSettings will create ApplicationSetting.current if not present which means we don't have to use &.. We can also more easily use stub_application_setting in specs

What are the relevant issue numbers?

Does this MR meet the acceptance criteria?

• 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 database guides
• [-] Link to e2e tests MR added if this MR has Requires e2e tests label. See the Test Planning Process.
• Security reports checked/validated by reviewer