Previously, this would require manually adding `?safe_mode=...` multiple times during the email-based login flow. `/u/admin-login` is often used when debugging a site, so it makes sense for this to be easier.
This commit introduces a new checkbox on the `/u/admin-login` screen. When checked, it'll set the safe_mode parameter on the `/email-login` link, and then pass it all the way through to the homepage redirect.