Fix Route validation when conflicting permanent redirects exist

Closes gitlab-com/support-forum#2883 and #41786

See merge request gitlab-org/gitlab-ce!16397

(cherry picked from commit d607f16f)

1dc30595 Revert "Revert "Fix Route validation for unchanged path""
c517788e Fix Rubocop offense
601c24d5 Add RedirectRoute factory
d4c8d1b7 Add changelog entry