add a simple login page & placeholder portal page

edit urls.py to use django's built-in login view, create a template for the
login page, edit settings.py so that users are redirected to '/portal/' on
login (unless they were redirected to the login page from somewhere else, in
which case they get redirected back to that page).
get rid of useless /auth/ url prefix because it was silly anyway
configure urls.py for the stock logout view & add template
...