Fix django auth login redirection

The django authentication decorator login_required, redirects to
settings.LOGIN_URL. If it is missing, it redirects to /accounts/login.

We did not have the setting enable, not a proper page where to
redirect. This cause users not logged in to see a 404 error when
accessing courseware directly, by using a link or a bookmark.

[#41499213]

common/djangoapps/student/views.py

@@ -333,6 +333,18 @@ def change_enrollment(request):
     return {'success': False, 'error': 'We weren\'t able to unenroll you. Please try again.'}
 
 
+@ensure_csrf_cookie
+def accounts_login(request, error=""):
+
+    return_to = ''
+
+    return render_to_response('accounts_login.html', {
+        'error': error,
+        'return_to': return_to
+    })
+
+
+
 # Need different levels of logging
 @ensure_csrf_cookie
 def login_user(request, error=""):