Fix redirects

This fixes the broken redirects (at least the ones that I could find).
1. firstboot redirects (and form action) are relative paths, so they should not include "firstboot".
2. Other redirects in auth, auth_page, and router are absolute paths, so they should include server_dir.
3. Other form actions are absolute paths, so they should include server_dir.

I haven't been able to test everything here yet, because of some issue that's preventing me from logging in to plinth.