[ui][view] Add missing Http404 import

Fixes #438

beat/web/ui/views.py

@@ -34,6 +34,7 @@ from django.contrib.auth.forms import PasswordChangeForm
 from django.contrib.auth.models import User
 from django.contrib import messages
 from django.http import HttpResponseRedirect, HttpResponse
+from django.http import Http404
 from django.contrib.auth.decorators import login_required
 from django.core.urlresolvers import reverse
 from django.db.models import Q