WIP: Allow sidekiq admin requests, regardless of root

What does this MR do?

Gitlab::::Middleware::ReadOnly::Controller#sidekiq_route? checks for sidekiq admin requests but failed to take into consideration if the GitLab installation was mounted at anything other than /, e.g. /gitlab.

What are the relevant issue numbers?

https://gitlab.com/gitlab-org/gitlab-ee/issues/6868

Does this MR meet the acceptance criteria?

• [ ] Changelog entry added, if necessary
• [ ] Documentation created/updated via this MR
• 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](https://about.gitlab.com/handbook/engineering/quality/test-engineering/).
• [ ] EE specific content should be in the top level /ee folder
• [ ] For a paid feature, have we considered GitLab.com plans, how it works for groups, and is there a design for promoting it to users who aren't on the correct plan?
• Security reports checked/validated by reviewer

Closes #6868 (moved)