chore: Cleanup Query Engine and Product query API

- Resolved merge conflicts in item_group.py
- Separate api.py file for product listing backend api
- Brought back ORM in query engine, handled missing cases (website item groups, etc)
- Return results from API in a descriptive manner, helps keep sanity in JS
- On toggling views store view preference in localStorage

erpnext/e_commerce/api.py

@@ -0,0 +1,49 @@
+# -*- coding: utf-8 -*-
+# Copyright (c) 2021, Frappe Technologies Pvt. Ltd. and contributors
+# For license information, please see license.txt
+
+import frappe
+from frappe.utils import cint
+
+from erpnext.e_commerce.product_query import ProductQuery
+from erpnext.e_commerce.filters import ProductFiltersBuilder
+from erpnext.setup.doctype.item_group.item_group import get_child_groups
+
+@frappe.whitelist(allow_guest=True)
+def get_product_filter_data():
+	"""Get pre-rendered filtered products and discount filters on load."""
+	if frappe.form_dict:
+		search = frappe.form_dict.search
+		field_filters = frappe.parse_json(frappe.form_dict.field_filters)
+		attribute_filters = frappe.parse_json(frappe.form_dict.attribute_filters)
+		start = cint(frappe.parse_json(frappe.form_dict.start)) if frappe.form_dict.start else 0
+		item_group = frappe.form_dict.item_group
+	else:
+		search, attribute_filters, item_group = None, None, None
+		field_filters = {}
+		start = 0
+
+	sub_categories = []
+	if item_group:
+		field_filters['item_group'] = item_group
+		sub_categories = get_child_groups(item_group)
+
+	engine = ProductQuery()
+	result = engine.query(attribute_filters, field_filters, search_term=search,
+		start=start, item_group=item_group)
+
+	# discount filter data
+	filters = {}
+	discounts = result["discounts"]
+
+	if discounts:
+		filter_engine = ProductFiltersBuilder()
+		filters["discount_filters"] = filter_engine.get_discount_filters(discounts)
+
+	return {
+		"items": result["items"] or [],
+		"filters": filters,
+		"settings": engine.settings,
+		"sub_categories": sub_categories,
+		"items_count": result["items_count"]
+	}