Add ability for providers to view a user cart and remove items from it

[Deep-Chill]

saas/api/billing.py:

• Add ActiveCartItemListCreateView and ActiveCartItemRetrieveUpdateDestroyView for cart item CRUD operations.
• Add UserCartItemListView for listing specific user's cart items.

saas/api/plans.py:

• Added missing import.

saas/api/serializers.py:

• Add ActiveCartItemSerializer with computed amount field. Not sure if it's worth creating a new serializer for just a single new field but didn't want to affect existing usages of CartItemSerializer.
• Add ActiveCartItemUpdateSerializer for update operations with read-only user and planfields for specific cart items.
• Add UserCartDataSerializer for specific user cart. This removes the User and "detail" fields from the ActiveCartItemSerializer(which inherits from CartItemSerializer), User because it would repeat the "user" field for each item and detail because I wasn't sure if it would be necessary for only GET requests.
• Add ActiveCartItemCreateSerializer for new cart item creation with specified user and plan querysets.

saas/static/js/djaodjin-saas-vue.js:

• Add Vue components for active carts and user carts with methods for item addition, removal, and updates in user carts.

saas/templates/saas/billing/cartitems.html:

• Template for rendering active carts.

saas/templates/saas/billing/user_cartitems.html:

• Template for rendering user's active cart.

saas/urls/api/provider/init.py:

• Add URL configuration for cart items.

saas/urls/api/provider/cart.py:

• Add URL patterns for cart item operations.

saas/urls/views/broker.py:

• Add URL patterns for active carts view and user active carts view.

saas/views/billing.py:

-Add ActiveCartItemsView and UserCartItemsView template views. testsite/urls.py:

-Update URL configuration for cart related requests.