Allow UrlValidator to work with attr_encrypted

What does this MR do?

Addresses some ~"technical debt" in the UrlValidator that prevents it from working with attr_encrypted

As seen in https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/21645#note_101482083

What are the relevant issue numbers?

Related to https://gitlab.com/gitlab-org/gitlab-ce/issues/51021

Does this MR meet the acceptance criteria?

• [-] Changelog entry added, if necessary
• [-] Documentation created/updated
• Tests added for this feature/bug
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides