Refactor subpage perm checks

What does this MR do and why?

• Fix bug where we would retain the subpage a user last navigated to. We'd keep serving this page even if they didn't have permission and would get them stuck in a no access error
• Refactor permissions checks on subpages. Tried to make it more understandable and dry.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• Have you checked this doesn't interfere/conflict/duplicate someone elses work?
• Have you fully tested your changes?
• Have you resolved any lint issues?
• Have you assigned a reviewer?
• Have you applied correct labels?