Redirecting admin page to login screen when logged out

Currently when I try to access https://hypothes.is/admin/ when logged out, I get a 'page not found' error. A better UX would be to redirect to: https://hypothes.is/login, then post login re-direct back to https://hypothes.is/admin/.

Steps to reproduce:
1. Log out of http://hypothes.is
2. Go to https://hypothes.is/admin/
3. You will see the page not found page (attached screenshot)

Suggested solution(s):
Not a bug, but confused me as a new support worker and would be better to have admin re-direct to a log-in screen or even a popup window containing login fields, or minimal implementation would be to provide a link to the login screen https://hypothes.is/login.

Start by reproducing the behavior at https://hypothes.is/admin/ while logged out, then inspect how that route handles unauthenticated access and the existing https://hypothes.is/login flow. Done means an unauthenticated visit reaches the login screen and, after login, returns to /admin/; at minimum, the admin page should provide a link to login.

Written by the indexing model from the issue text.