counter: make click page dynamic to avoid repetitive loading

This makes the whole click page load only once for a normal click
workflow. The current basket is now rendered client side with Vue.JS,
and the backend view is able to answer with JSON if asked to.

This should lighten the workflow a lot on the client side, especially
with poor connectivity, and the server should also feel lighter during
big events, due to far less complex Jinja pages to render.

counter/views.py

@@ -38,7 +38,7 @@ from django.views.generic.edit import (
 from django.forms.models import modelform_factory
 from django.forms import CheckboxSelectMultiple
 from django.urls import reverse_lazy, reverse
-from django.http import HttpResponseRedirect, HttpResponse
+from django.http import HttpResponseRedirect, HttpResponse, JsonResponse
 from django.utils import timezone
 from django import forms
 from django.utils.translation import ugettext_lazy as _
@@ -48,6 +48,7 @@ from django.db import DataError, transaction, models
 import re
 import pytz
 from datetime import date, timedelta, datetime
+from http import HTTPStatus
 from ajax_select.fields import AutoCompleteSelectField, AutoCompleteSelectMultipleField
 from ajax_select import make_ajax_field
 
@@ -357,6 +358,34 @@ class CounterClick(CounterTabsMixin, CanViewMixin, DetailView):
     pk_url_kwarg = "counter_id"
     current_tab = "counter"
 
+    def render_to_response(self, *args, **kwargs):
+        if self.request.is_ajax():  # JSON response for AJAX requests
+            response = {"errors": []}
+            status = HTTPStatus.OK
+
+            if self.request.session["too_young"]:
+                response["errors"].append(_("Too young for that product"))
+                status = HTTPStatus.UNAVAILABLE_FOR_LEGAL_REASONS
+            if self.request.session["not_allowed"]:
+                response["errors"].append(_("Not allowed for that product"))
+                status = HTTPStatus.FORBIDDEN
+            if self.request.session["no_age"]:
+                response["errors"].append(_("No date of birth provided"))
+                status = HTTPStatus.UNAVAILABLE_FOR_LEGAL_REASONS
+            if self.request.session["not_enough"]:
+                response["errors"].append(_("Not enough money"))
+                status = HTTPStatus.PAYMENT_REQUIRED
+
+            if len(response["errors"]) > 1:
+                status = HTTPStatus.BAD_REQUEST
+
+            response["basket"] = self.request.session["basket"]
+
+            return JsonResponse(response, status=status)
+
+        else:  # Standard HTML page
+            return super().render_to_response(*args, **kwargs)
+
     def dispatch(self, request, *args, **kwargs):
         self.customer = get_object_or_404(Customer, user__id=self.kwargs["user_id"])
         obj = self.get_object()