Read-only controller doesn't account for relative paths for /admin/sidekiq route

Found while looking at https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/6248#note_86991051:

For configurations using relative URLs, the request.path includes the relative path. For example, if external_url is set to http://geo-secondary/test, the request.path may be /test/admin/sidekiq.

Currently the use of start_with? in the read-only controller would not properly match.