If an already authenticated user visits /accounts/login/,
Healthchecks will now redirect them to their dashboard
instead of showing the login form.

This fixes a security issue:
- attacker can crafts a redirect URL to an external site
- attacker gets victim to click on it
- victim logs in
- after login, Healthchecks redirects victim to the external site

The _allow_redirect function now additionally
requires the redirect URL is relative (has no scheme or domain).

Fixes: #354