Avoid checking the user format in every url validation

What does this MR do?

In https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/18686 we make our policy with url validations more strict, checking also the user format if it is present in the url. But, it seems there are cases where the user can have special characters (See https://gitlab.com/gitlab-org/gitlab-ce/issues/47526).

In this MR we make the user format validation optional, configured when we in the validator.

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• API support added
• Tests added for this feature/bug
• Conform by the code review guidelines
  • Has been reviewed by a UX Designer
  • Has been reviewed by a Frontend maintainer
  • Has been reviewed by a Backend maintainer
  • Has been reviewed by a Database specialist
• Conform by the merge request performance guides
• Conform by the style guides
• If you have multiple commits, please combine them into a few logically organized commits by squashing them
• Internationalization required/considered
• End-to-end tests pass (package-and-qa manual pipeline job)

What are the relevant issue numbers?

Fixes #47526 (closed)