Improve 404 page to help users understand that content may be restricted

Summary

When users are logged in but attempt to view a space they do not have access to, they are shown an unhelpful 404 screen that states "Oops, you've found a dead link".

Steps to reproduce

1. Create a space restricted to User A, copy a link to that space
2. Log in as User B and attempt to access the URL copied in Step 1
3. Observe the following 404 screen:

Suggestion

This screen should help users understand that they may be attempting to access restricted content. Take this 404 screen as an example (shown for both a real 404 response, and the response when a user cannot view a page due to inherited restrictions):