Add an endpoint to list products of a vendor.
Add endpoint to list products of vendor + some bug fixes.

- add REST endpoint to list products of a vendor.
- Replace image_path with ImageField (models).
- add MEDIA_ROOT and MEDIA_URL in settings for user-uploaded images.
- add vendor and product models on admin page.
- removed user serializer from VendorSerializer. django REST framework automatically match model fields for foriegn keys.
- removed AddVendorSerializer -> duplicate of VendorSerializer.