[fix bug 976099] Redirect to current page after logging in.

This is OK but let's make it beautiful:
 1. instead of `/` use settings.LOGIN_URL
 2. instead of string formating to build a URL, use urlparams


```
from funfactory.helpers import urlparams  # move this input at top
next_url = urlparams(reverse('main'), next=request.path)
return HttpResponseRedirect(next_url)
```