Bug with initial-page when logging in

Reported by a user, thank you:

When you open the app in the browser at /, you get redirected to /app and the webapp loads. It realizes you are not logged in so it redirects you to the login page with /app/home as referrer. And because the referrer overrides the `inital-page` option (Login/Update.elm L83), you get redirected to the home-page after logging in. I get that the referrers priority should be higher, so the login works when deep-linking to unpublished shares and such. The only ways I found to get to the login page without a referrer is after logout or when entering the url directly into the browser address bar.

Start at Login/Update.elm line 83 and reproduce the navigation from / through /app to the login page. Trace how the /app/home referrer and the initial-page option are handled, then verify that logging in follows the intended destination for both the root entry flow and deep links.

Written by the indexing model from the issue text.